Portuguese Subjunctive Guide
The subjunctive (O conjuntivo) can be tricky to grasp for English speakers, as it's rare in our language. I'm putting this guide together in the hopes that it will help those who struggle with this aspect of Portuguese.
What is the subjunctive?
The subjunctive is a verb mood. What does that mean? A verb mood, of which there are four, shows the meaning behind a verb, not the time at which it occurred - the subjunctive ≠ a tense. The four verb moods which exist are the indicative, the conditional, the subjunctive, and the jussive. The indicative is the easier form which is learnt first - used for things like facts or definite situations, like “He was here”. The conditional is used for situations which are dependent on other situations to be able to occur, like “I would go, if I had the time“. The jussive is the form used for the “Let's ...“ and imperative forms of the verb, like “Let's eat” and “Eat!”. The subjunctive, however, is used in situations of doubt, desire, opinion, and others, which will be shown later.
How to conjugate the subjunctive - Present Tense
To conjugate the subjunctive in the present tense, take the eu form of the verb, remove the final -o, then add the following endings:
So, let's look at these endings on some verbs:
Due to orthographical rules in Portuguese, the spelling of some verbs changes when they are put into the subjunctive. Here are the endings that change, with an infinite and the eu form of the subjunctive as an example:
There are unfortunately irregular verbs in the subjunctive. These are:
- Ser: Sej- (For example, “Eu seja“, “Eles sejam“)
- Estar: Esteja- (For example, “Tu estejas“, “Nós estejamos“)
- Dar: D-; The final e takes a circumflex accent (For example, “Ele dê“,“Nós dêmos“)
- Saber: Saib- (For example, “Eu saiba“, “Elas saibam“)
- Haver: Haj- (For example, “Ele haja“)
- Ir: V-; Eu, Tu, Ele/Ela/Você: the a takes an acute accent. The Nós & Eles/Elas/Vocês conjugation is the same as the indicative (For example, “Ele vá“, “Nós vamos“)
- Querer: Queir- (For example, “Eu queira“, “Elas queiram“)
How the present subjunctive is used
The use of the present subjunctive can be hard to get the hang of at first, as it appears so seldom in English (an example being “I suggest that he go“). If you've already encountered the subjunctive in other languages like Spanish, it will be a bit easier, as many of the “subjunctive phrases” will carry over to Portuguese. Likewise, getting familiar with the subjunctive in Portuguese first will help when it comes to using it in other languages.
If there is doubt within a situation, the subjunctive is used. For example, phrases like “Não acredito que...“ (I don't believe that...), “Duvido que...“ (I doubt that...), “Não tenho a certeza de que...“ (I'm not sure that...) all use the subjunctive. The use and wording of phrases will depend on the region that you’re in. Remember that any phrases where there isn't doubt don't use the subjunctive, for example “Acho que...“ (I think that...) and “Não duvido que...“ (I don't doubt that...). Let's look at some example sentences - words in Italics are “subjunctive phrases“, and words in Bold are verbs in the subjunctive.
- Não acredito que ele aprenda dez línguas à vez - I can't believe he is learning ten languages at once
- Não tenho a certeza de que isto seja bom - I'm not sure this is good
- Duvido que estejas a dizer a verdade - I doubt you're telling the truth
And here are these “subjunctive phrases“ flipped, to show them with the indicative:
- Acredito que ele aprende dez línguas à vez - I can believe that he is learning ten languages at once
- Tenho a certeza de que isto é bom - I'm sure this is good
- Não duvido que estás a dizer a verdade - I don't doubt you’re telling the truth
If there is a wish or an order, something which may not end up happening, the subjunctive is used. Some phrases which require the subjunctive are “Quero que...“ (I want (that)...), “Espero que...“ (I hope (that)...), and “Desejo que...“ (I wish (that)...). Let's look at some example sentences of this use:
- Elas desejam que tudo corra bem - They wish that everything will go well
- Não queremos que chova amanhã - We don't want it to rain tomorrow
- Os meninos esperam que não haja escola na quinta-feira - The children hope that there's no school on Thursday
3. Impersonal statements
Statements like “É bom que...“ (It's good that...), “É fácil que...“ (It's likely that...), and “É improvável que...“ (It's unlikely that...) use the subjunctive. However, remember that the subjunctive is used where there is uncertainty - so statements like “É certo que...“ (It's certain that...) wouldn't be used with the subjunctive. Here are some example phrases:
- É bom que as pessoas saibam os perigos - It's good that people know the dangers
- É improvável que eles ganhem a partida - It's unlikely that they'll win the game
- É estranho que ele não se lembre - It's strange that he doesn't remember
Here is a list of some phrases which use the subjunctive:
4. Direct effect
If an action affects you directly (e.g. “I don't like that...“), the subjunctive is used. Some phrases which use the subjunctive in this way are “Perturba-me...“ (It bothers me that...), “Gosto que...“ (I like that...), and “Entristece-me que...“ (It upsets me that...). Some examples are:
- Perturba-me que o voo seja cancelado - It bothers me that the flight is cancelled
- Não gostamos que haja tanta gente - We don't like that there are so many people
- Entristece-me que você esteja tão infeliz - It upsets me that you’re so unhappy
5. Fixed expressions
Some fixed expressions use the subjunctive. Some examples of such expressions are “Embora...“ (Although...), “Não obstante que...“ (Despite...), and “A não ser que...“ (Unless...). Here are some examples:
- Não obstante que esteja a chover, vamos nadar - Despite the fact that it’s raining, we are going to swim
- Embora ele tenha fome, não come o jantar - Although he is hungry, he doesn’t eat dinner
- Caso você não coma carne, há pratos vegetarianos - If you don’t eat meat (In the case of you not eating meat), there are vegetarian dishes
- Não vou ao estrangeiro a não ser que conheça alguém no país - I don’t go abroad unless I know someone in the country
6a. Commands - Tu (Portugal)
When saying a negative command to someone you address as “tu“, the tu form of the present subjunctive is used:
- Não fales assim! - Do not speak like that!
- Não te esqueças da tua bagagem de mão - Don't forget your hand luggage
6b. Commands - Você & Vocês
For commands said to someone you address as “você“ (both positive and negative), the ele/ela/você form of the present subjunctive is used:
- Ponha a bolsa aquí - Put the bag here
- Não fume no carro - Do not smoke in the car
Likewise, commands to a group you address as “vocês“ use the eles/elas/vocês form of the present subjunctive:
- Aprendam comigo! - Learn with me!
- Por favor sejam pacientes - Please be patient
- Não percam a chave - Do not lose the key
- Não usem o elevador em caso de incêndio - Do not use the lift in case of fire
“Oxalá“ comes from the Arabic “Insha 'allah“, and means “Let's hope that”, or “I hope that”. An alternative is “Tomara“, which means the same thing. Both use the present subjunctive:
- Oxalá não percamos nosso voo - Let's hope we don't miss our flight
- Tomara seja bom! - Let's hope it's good!
The last use of the subjunctive is starting a sentence or clause with “Que“, then using the subjunctive. This is used to show desires and thoughts. For example:
- Que nunca partamos! - Let's never leave!
How to conjugate the subjunctive - Imperfect Tense
The subjunctive also exists in the imperfect tense. To conjugate it, take the eles/elas/vocês form of the verb in the preterite tense and remove the -aram/-erem/-irem, then add the following endings:
Here are the endings on the verbs we looked at for the present tense:
How the imperfect subjunctive is used
The imperfect subjunctive is used, for the most part, in the same cases as the present subjunctive, except for the fact that the preceding phrases are either in the conditional or imperfect tenses. So while the present subjunctive would be used with “Quero que...“ (I want (that)...), the imperfect subjunctive would be used with “Gostaría que...“ (I would like (that)..) and “Queria que...“ (I wanted (that)...). This carries across for all aspects:
- Não pensei que ele aprendesse espanhol - I didn't think that he learnt Spanish
- Esperávamos que todos o vissem - We hoped that everyone saw it
- Seria improvável que ninguém viesse - It would be unlikely that no one came
- Gostaria que lesses o livro - I’d like you to read the book
Se + Conditional
One feature of the imperfect subjunctive is the “Se + Conditional“ sentence. This is used to state hypothetical actions - which is why the subjunctive is used. This type of sentence is the only sentence where the subjunctive is still fairly identifiable in English - “If I were rich, I'd buy a mansion“. This type of sentence works exactly the same in Portuguese:
- Se você não tivesse de trabalhar, o que faria? - If you didn't have to work, what would you do?
The sentence structure can also be flipped, starting with the conditional clause and then using the imperfect subjunctive. For example:
- Eu falaria mais em português se tivesse mais confiança - I'd speak in Portuguese more if I had more confidence
“Como se“ is the translation of “As if“ in Portuguese, which always uses the imperfect subjunctive. It works in the exact same ways as in English:
- Foi como se nunca partisse - It was as if he never left
- Como se apenas eu gostasse daquilo! - As if it were only I who liked it!
Perfect and Pluperfect Subjunctive
The perfect and pluperfect subjunctive tenses also exist in Portuguese. These are formed by either using the present subjunctive of Ter + Past Participle, or the imperfect subjunctive of Ter + Past Participle. For example:
- É incrível que elas tenham ganhado a competição – It's incredible that they won the competition
- Se tivéssemos sabido que o tempo seria tão mal, não teriamos ido – If we'd known the weather would be so bad, we wouldn’t have gone
How to conjugate the subjunctive - Future Tense
Portuguese is the only Romance language to still have the future subjunctive active. To conjugate the future subjunctive, take the eles/elas/vocês form of the verb in the preterite tense and remove the final -am, then add the following endings:
Here are these endings on the verbs we looked at earlier:
How the future subjunctive is used
The future subjunctive is used to talk about future events with uncertainty. In most other languages, either the future indicative or present subjunctive is used in these cases.
When “Quando“ is used with a future action, the future subjunctive is used:
- Quando sair, irei ao parque e depois ao banco – When I go out, I will go to the park and then to the bank
Remember that if “Quando“ isn’t referring to the future, the future subjunctive isn’t used:
- Quando fui ao banco, havia muita gente - When I went to the bank there were a lot of people
- Quando saio lembro-me sempre de levar as minhas chaves - When I go out I always remember my keys
Like “Quando“, if “Se“ is used with a future action, the future subjunctive is used:
- Se vocês não pararem de falar, teremos de partir - If you all don't stop talking, we'll have to leave
“Enquanto“, meaning “As long as“, or “While“, used with a future action uses the future subjunctive:
- Enquanto estiveres aqui, estarei feliz - As long as you are here, I'll be happy
Compare this to the same phrase with the present indicative:
- Enquanto estás aqui, estou feliz - As long as you are here, I'm happy
The first sentence is referring to the future, whereas the second is referring to the present.
Assim que.../Logo que...
Both “Assim que“ and “Logo que“ mean “As soon as“. Like “Quando“, “Se“, and “Enquanto“, when referring to a future action they use the future subjunctive:
- Assim que eu tiver feito todo o meu trabalho, poderei ir convosco - As soon as I've finished all my work I'll be able to come with you
- Ela vai-nos chamar logo que se recuperar - She is going to call us once she is better
I hope this helps!/Espero que isto ajude!
Check out my other subjunctive guides!
I would like to suggest a few adjustments for this excellent post. Its very good to have people who wish to help and bring such a detailed guide to us.
First, this link is a very good resource for the English subjunctive. If you understand this, you are half the way ahead :)
There may be a difference between Portugal and Brazil here, I can only speak for Brazil, though.
1 - Doubt
In Brazilian Portuguese, we don't know the expression "à vez", we use "de uma vez (só)" instead.
These two examples prefer the subjunctive:
- Acredito que ele aprenda dez línguas de uma vez - I believe that he learns ten languages at once (there may still be doubt here)
- Eu não duvido que estejas dizendo a verdade - I don't doubt you’re telling the truth (this prefers to follow what the verb "duvido" asks for)
In the first one, using the indicative is assuming and accepting he actually learns 10 languages. Ex: Someone tries to convince you, and you actually accept that as true.
Using the subjunctive is to believe he probably does.
6 - Commands
Although having the same form, imperative conjugations shall not be seen as subjunctive conjugations.
But you can use it as a very good trick to remember.
8 - Que...
In Brazil, we don't understand the expression "Que não os entenda?"
We would use one of the following and others:
- Ele não fez os deveres. Talvez não os entenda? (Perhaps he doesn't understand them?)
- Ele não fez os deveres. Será que não os entende? (Could it be that he doesn't understand them?)
How the imperfect subjunctive is used
The following examples must have their forms/translations adjusted:
- Era/Seria bom que vocês sobrevivessem - It would be good if you were to survive
- É bom que vocês sobrevivam - It's better for you to survive / You'd better survive
- É bom que vocês sobreviveram - It's good that you survived
(É bom que vocês sobrevivessem - Tense mismatch)
Não acredito que ele partisse sem dizer «Tchau» - I don't believe that he would leave without saying “Goodbye”
- Não acredito que ele partiu sem dizer «Tchau» - I don't believe that he (has) left without saying “Goodbye”
Perfect and Pluperfect Subjunctive
Adjustments (for BR Portuguese):
- Se tivéssemos sabido que o tempo seria/estaria/ficaria tão ruim, não teriamos ido – If we'd known the weather would be so bad, we wouldn’t have gone
Well.... that statement can be too vague....
There are keywords and keyverbs that ask for the subjunctive.
Also, conditional sentences and a few other constructions.
It tends to be about "hypothetical" cases, but not strictly.
The imperfect subjunctive can be used either in past cases or in hipothetical conditionals.
The explanation from the past subjunctive skill quite sums up the way I currently see it:
The Past Subjunctive
The past subjunctive talks about actions in the past that are uncertain and also about hypothetical things in general.
In English, you can find past subjunctive in "if clauses":
- If I were you, I would stay here = Se eu fosse você, eu ficaria aqui
The conditions for using the past subjunctive are very similar to those for using the present subjunctive. There is just a difference in tenses.
But here, a very important case comes into play: the conditionals!
Conditionals with "if"
One of the most important usages of the past subjunctive is for creating conditionals. You will notice also that the "if" keyword (se) is present in conjugation tables.
- Se eu fosse rico, compraria um barco = If I were rich, I'd buy a boat
- Nós iríamos à praia se não estivesse chovendo = We would go to the beach if it weren't raining
- Se eles gostassem de biscoitos, eu lhes daria uma caixa = If they liked cookies, I'd give them a box
Optionally, there is the informal possibility of using the imperfect past in the main clause:
- Se eu fosse rico, comprava um barco
Some verbs asking for the subjunctive
Examples of verbs expressing "hope/doubt" in past:
- Eu esperava que você voltasse = I was expecting you to come back
- Duvido que fosse verdade = I doubt it was true
- Não creio que fosse possível = I don't believe/think it was possible
- Eu não acreditava que fosse possível = I didn't believe it was possible
- Eu acreditava que as coisas fossem melhorar = I believed things were going to get better
- Ela desejava que tudo acabasse bem = She was hoping everything would end well
Verbs giving orders:
- Ordenei que os prisioneiros fossem libertados = I commanded that the prisoners be freed
- Eles pediram que tirássemos os sapatos antes de entrar = They asked us to take off our shoes before entering
- Sugeri que eles esperassem a tempestade passar = I suggested that they wait for the storm to pass
- Ela insistiu que eu provasse um pedaço do bolo = She insisted that I try a piece of the cake
Keywords asking for the subjunctive
Here too, some keywords will ask for the past subjunctive just like they do in present tense. But now, a very important keyword comes into play: "if"; allowing us to create the most common conditionals. (Note that "if" is present in the conjugation tables for past subjunctive)
- Caso fosse verdade, então deveríamos nos preocupar = If it were true, then we should worry
- Embora fôssemos grandes, não podíamos alcançar o céu = Although we were big, we couldn't reach the sky
- Eu faria isto, mesmo que ele não concordasse = I'd do this, even if he didn't agree
- Ainda que eu quisesse, não poderia = Even if I wanted to, I couldn't
- Eu acharia você, nem que eu precisasse dar a volta ao mundo = I would find you, even if I needed to circle the world around.
- Ele tomou o remédio para que melhorasse = He took the medicine in order to get better
- Ela fez tudo de modo que não deixasse vestígios = She did everything in a way that didn't leave traces
If you're not picky, you can let the difference pass unnoticed.
But if you are:
With "que": it expresses more like a wish, it may sound somehow like a command. It can sound close to "You'd better survive", although the present tense (É bom que vocês sobrevivam) is the most suitable for that command-like sentence. Another possibility is "It would be good for you to survive".
With "se": it expresses that I'd like the result if you happen to survive. It gives more chance and less importance to the case of not surviving.
The following seems to be wrong:
However, if a structure like the “impersonal statements“ is used to refer back to a past event, this will remain in the present tense:
However, the following examples use the past tense:
- É bom que vocês sobrevivessem - It's good that you all survived
- Não acredito que ele partisse sem dizer «Tchau» - I don't believe that he left without saying “Goodbye”
And then about the future subjunctive:
To conjugate the future subjunctive, take the eles/elas/vocês form of the verb in the preterite tense and remove the final -am, then add the following endings:
But then the examples take the infinitive as a base.
And I'd like to qualify this statement:
Portuguese is the only Romance language to still have the future subjunctive active.
The future subjunctive is not used in the day to day language in Spanish, but it can be found in famous bible verses, in literature, in the legal realm and some popular expressions. As a native Spanish speaker I find useful to compare the Spanish future subjunctive with its equivalent in Portuguese.
⇒ Yes, the verb in the subjunctive uses the past tense, but the “impersonal statements“ are in the present tense.
⇒ The infinitive isn't used as a base. Let's take “Falar“ - “To speak“ as an example. Here are the conjugations in the preterite:
Remember, to get the future subjunctive stem, you remove the -am from the Eles/Elas/Vocês form, which leaves us with “Falar“, which I've indicated with bold text. This just happens to coincide with the infinitive. If you have an irregular verb however, like “Querer“ - “To want“, you end up with the following conjugations:
With this verb, you can see that the future subjunctive stem, “Quiser“, and the infinitive “Querer“ are different. For example:
- Se vocês quiserem ir à universidade, vocês terão de estudar - If you want to go to university, you'll have to study
⇒ Notice the word “active“. Yes, Spanish has the future subjunctive, but it's not really used on a day-to-day basis, is it? Whereas in Portuguese, it is. For example, in my phone's settings, it says “Se não estiverem redes conhecidas disponíveis, terá de selecionar uma rede manualmente“. If my phone were in Spanish, you wouldn't (and don't) see the future subjunctive anywhere in the settings app.
I see where I was wrong. Thank you, it's clearer now.
Yes, Spanish has the future subjunctive, but it's not really used on a day-to-day basis, is it? Whereas in Portuguese, it is.
It's not used on a daily basis, it's true. I just wanted to point that it is known, and still used in some contexts, in Spanish.
Two of your examples break the normal "sequence of tenses":
É bom que vocês sobrevivessem.
Não acredito que ele partisse sem dizer «Tchau.»
The present perfect subjunctive describes a possible completed action in the past and is triggered by a present tense verb in the main clause.
É bom que vocês tenham sobrevivido. = It's good that you've survived.
Não acredito que ele tenha partido sem dizer "Tchau." = I don't believe that he's left without saying "Goodbye".
You probably won’t see this since this post is from a few years ago, but I have hope.
So there are 4 tenses of the subjunctive (excluding the future, which is slightly different), right? Like this?:
Duvido que eles comam o arroz. (Present subjunctive)
Duvido que eles tenham comida o arroz. (Perfect subjunctive)
Duvidei/duvidava/ duvidaria que eles comessem o arroz. (Past subjunctive)
Duvidei/duvidava/duvidaria que eles tivessem comido o arroz. (Pluperfect subjunctive)
If I wanted to talk about the future, such as in the sentence ‘I will doubt that they ate the rice’, would I say ‘Eu vou duvidar/duvidarei que eles comeram o arroz’, not using the subjunctive?
Also, does Duolingo teach the perfect subjunctive because there only seems to be a pluperfect subjunctive skill.
Subjunctive is a killer hehehe but it's such a beautiful mood, and once you master it, I guarantee that you will impress native speakers.
So before diving into this, I want to ask you if you know the difference between Tense and Aspect. If you don't, I'd recommend you to read this: https://courses.lumenlearning.com/boundless-writing/chapter/verbs/
In a nutshell: "Tense indicates when the action expressed by a verb takes place. The three simple tenses are past, present, and future. "
“Aspect refers to the timing of the verb. More specifically, it addresses whether the action occurs in a single block of time, continuously, or repetitively. All verbs have both tense and aspect. Verbal aspect consists of simple, progressive, perfect, or perfect progressive, where each refers to a different fabric of time."
"Verb mood is to the “attitude” of the verb. More specifically, “mood” refers to the degree of necessity, obligation, or probability. Is it a statement of fact? Is it a command? Mood can be expressed in any verb tense." - and I would add: with any aspect.
In Portuguese, Subjunctive mood has 3 tenses: past, present, and future (as opposed to, let's say, French and Spanish, whose subjunctive mood have only 2 tenses). It can also be expressed in different aspects: comam (simple), tenham comido (perfect), tivessem comido (perfect), estejam comendo (progressive), estiverem comendo (progressive).
Tense and aspect always come together, so to be more precise: comam (simple present), tenham comido (present perfect), tivessem comido (past perfect), estejam comendo (present progressive), estiverem comendo (future progressive). Note: Portuguese Mais-que-perfeito (pluperfect) correspond to English past perfect.
I hope this intro will help you to understand the intricacies of Portuguese general conjugation.
Now, answering your question about the future. The sentence "Eu vou duvidar que eles comeram o arroz" is correct and should not use the subjuntive. See, the doubt (duvidar) will come in the future about an action (comer) fully completed in the past. In my native ears, this sentence sounds like that you know that they ate, but you will doubt it anyways. Thus, the use of the indicative is necessary, there's no need of the beautiful doubtful shade of the subjunctive mood.
I'll give some examples on how this sentence can be changed to accommodate the subjunctive:
(1) Eu vou duvidar quando eles comerem. (2) Enquanto eles estiverem comendo, eu vou duvidar.
In sentence 1, they haven't eaten yet, in fact, you don't even know if they will eat, but when they do, you'll be ready to doubt it. Do you see now the need of the subjunctive? there's a uncertain action to happen in the future, which triggers the subjunctive. It's not always and it's not the only trigger, but "Quando + future action" is a great trigger for the future subjunctive since we can never know for sure anything about the future.
In sentence (2), same thing, you don't know if they will eat, but if they do and while they're doing it (that's why we need future progressive here), you will doubt it.
I hope these examples can make it more clear the usage of the subjunctive.
Keep pushing and don't discourage :D
Check my explanation above, and you will understand. Não acredito que ele partiu sem dizer tchau. The action of him leaving is complete, there's no doubt, it happened, so no subjunctive here. But if you say: Acredito que ele não tenha partido sem dizer tchau. Now you don't know for sure. You're just guessing, and then, you need the subjunctive. :D
As a native speaker (Brazilian), I'd be ok with the second example if it had a different translation, but I wouldn't agree with the first.
Não acredito que ele partisse sem dizer tchau = I don't believe he would leave without saying goodbye.
É bom que vocês sobreviveram = It's good you survived
- Seria/Era bom que vocês sobrevivessem = It would be good if you were to stay alive.
I'm having trouble with the use of the subjunctive in phrases like this:
Eu pensei que ela tivesse usado todo o dinheiro.
The translation proposed by Duolingo is:
I thought she had used all the money.
And in Spanish:
Yo pensé que ella hubiese usado todo el dinero.
Why use the subjunctive when a past tense of the indicative is enough?
In Spanish I would say:
Yo pensé que ella había usado todo el dinero.
Is there a rule for this?
I found somewhere a rule that says don't use subjunctive for verbs achar, pensar, acreditar, etc. when there is no doubt. In that case perhaps it could be "Eu pensei que ela tinha usado todo o dinheiro." However, in cases of being mistaken, the subjunctive is used when there is a clause that corrects or reveals the error. For example: "Eu pensei que ela tivesse usado todo o dinheiro, mas não fez isso." Maybe someone can shed more light on this rule.
Amazing article! Just a small adjustment. The verb "haver" is never conjugated. It only exists in the third person. So "tu hajas" doesn't exist in modern Portuguese. This verb only exists in its impersonal forms, so in subjunctive mood, we may only find "haja": Eu quero que haja mais dias de férias. (I want that there are more vacation days).
After studying Portuguese for over 4 years and being in Brasil 14 times I don’t understand this at all and it terrifies me. Verbs are difficult enough for me so adding more difficult concepts make learning it seem impossible to me. I still can’t understand how people can learn Portuguese with the struggle I have learning it. Thanks though. I appreciate your work.
Hi, consider this. "I don't want you to be a liar." which is literally translated, "I not want that you be a liar." Notice the present subjunctive "be" used even in English. Well if we said, "Não quero que você ser um mentiroso." It contains the words: você ser um mentiroso. So you could be very offended and say, I am a liar?? That way of writing would declare it as a fact. But instead, a different form of the verb is used to mean just a possibility: Não quero que você SEJA um mentiroso.
SEJA = possibly be
Subjunctive LIFTS THE MEANING AWAY FROM FACT. There are many rules and details for subjunctive, so it can be better to just get used to them one by one. Study "Talvéz". It's clear that such statements cannot be factual.
Talvéz ele ESTEJA com fome. Maybe he is hungry. Talvéz ele ESTIVESSE com fome. Maybe he was hungry.
The future tense with talvéz is usually said the same as present tense, or it's possible to say: Ele vá estar com fome. All subjunctive has a similar effect. Another easy expression is the future subjunctive "quiser".
QUISER = possibly want
If we said, "Se você quer jantar, eu vou cozinhar." then we say for a fact that you want dinner. But instead we say "Se você QUISER jantar, eu vou cozinhar." Cheers :)