"I study with my friend on Sundays."
Because it is a compound verb made by adding a noun to the verb します - this is very common in Japanese. 勉強します - to study, そうじ します - to clean, りょうり します - to cook, けんぶつ します - to sightsee or to go sightseeing. を is not used because べんきょう is not the object of the verb - it's part of the verb. Noun + する = compound verb.
Some things to think about whether を should be used:
1) Not all nouns can become "suru verbs". If you look on jisho.org, you'll see that "benkyou" is listed as a "suru verb", meaning 勉強 (benkyou) is a noun meaning "study" and 勉強します (benkyou shimasu) is a verb meaning "study". The word "tenisu" is not listed as a "suru" verb, so テニス (tenisu) is a word that means "tennis", but in Japanese you can't just make "tenisu" into a verb, you have to say "do tennis (play tennis)", テニスをします (tenisu o shimasu), where "tenisu" is the object of the verb and needs to be marked by を.
2) You can say 勉強します (benkyou shimasu) or 勉強をします (benkyou o shimasu), but the grammar and nuance is slightly different. 勉強します is a verb meaning "study". 勉強をします has "benkyou" as the object of the verb "shimasu", making the meaning "do studying".
3) People drop particles in speech all the time, even though it is not grammatically correct.
Just a note that we can say 買い物する (jisho.org is not correct). https://dictionary.goo.ne.jp/jn/37086/meaning/m0u/%E8%B2%B7%E3%81%84%E7%89%A9/
Basically most of the action nouns can be used as [object]+を+[noun]+する or [object]+の+[noun]+をする if it is transtive
Thank you for pointing that out. I'm a bit confused, though, because I thought kaimono was a suru verb and brought it up in another thread, and another user looked it up in their dictionary and said it wasn't listed as a suru verb.
hmm 買い物する sounds natural enough to me so I am sure the usage is ok. I tried to find out an example where the noun + する is natural to me but the dictionaries say otherwise. An example is デュエル (duel) where I find people say デュエルする (mostly with all sorts of card games). On the other hand for the word テニス (tennis), I feel unnatural to say テニスする (The goo dictionary does not say it can, so it is good). Well I think we can stick to any dictionary but just bear in mind it is more conservative that what people are using.
My issue is that everything sounds okay to me without を because I'm so used to native speakers dropping particles, so I'm really trying to understand the correct usage. I wonder if 買い物 didn't used to be a する verb but has become one...
Sports are a great example of non-suru nouns, so I'll edit my original post to avoid confusion, thanks for all your help!
When you want to say something like "every Sunday" is it correct to say "毎日曜日(まいにちようび)"?
And if not, what is the correct way of saying that something happens every week on the same day?
"I eat pizza every Sunday" = 毎日曜日ピザを食べます？？ は 日本語 で何と言いますか？
毎日曜日 is not used often.
毎週日曜日 is better.
Same problem here. I guess it's just unnatural to say the 私の referring to your own friend, as people will guess you don't learn with THEIR friend although you don't know them. It's more likely to add a (someone)の when you're not talking about your own friends ore stuff.
Anyway, I think Duo should accept it, as it is not grammatically incorrect - and when you're hovering over the "my", it still makes you think that you should add 私の (as the Japanese lessons here are still not developed well enough to differ translation of words in different context, as you can see with the kanji pronunciation).
を (instead of と) here would be incorrect. を follows the direct object of the verb (ie. in this case it would be what you are studying), however there is no direct object in this sentence - the sentence does not state what you are studying with your friends, just that you study with friends on Sunday. If you were to replace と with を you would make your friends the direct object of the verb and the sentence would mean I study my friend(s) on Sundays. と follows nouns to show whom you are performing an action WITH - in this case, the speaker states that they study WITH friends on Sundays. Please also note my and KeithWong's comments above in response to a query with a similar mistake.
Duo didn't accept your sentence because they have a specific sentence and specific grammatical constructions/words in mind that they are trying to teach with that sentence in mind. This is very common with Duo - the other day someone asked why instead of changing the ending of a verb so that it meant 'and' and could join two sentences together Duo wouldn't accept joining two sentences together using そして. I responded that likely it is because Duo is trying to teach us that you can change the endings of verbs so that they act like a conjunction and can be used to join sentences together, whereas if Duo wanted to teach how to join sentences together using actually conjunctions then their sentence would be acceptable. Make sense?
と following a noun - specifically a noun that is a living creature - human/animal indicates that you are carrying out a certain action WITH that noun. For example if you were walking with your dog then と would follow dog to indicate that you were walking with your dog - 犬と 散歩します, or if you were talking on the phone with your mum - 毎日母と でんわで 話します - I talk on the phone with my mum every day - here と follows 母 to indicate that 'mum' is the person that you are talking with. Also, scroll up and have a look through all the comments above, と and its function in this sentence has been queried and discussed at length in the above comments.
勉強します is the verb "to study" and you've placed a "with my friend" in the middle of it.
勉強 is a する verb but 勉強友達と is not
If you're going to separate the verb into a noun and a verb to say "to do a study" it would be 勉強をします but it still sounds the most natural to have the direct object near the verb.
-masu form can be the future tense, or it can be the present tense when talking about a habit. The -te form means that you are doing something right now.
勉強します。 (benkyou shimasu)
[future] I will study.
[present] I study (as a regular habit).
勉強しています。(benkyou shite imasu)
[present progressive] I am studying (right now).
Have you read the previous comments? https://forum.duolingo.com/comment/23250371?comment_id=32261680
友達「とは」勉強します →i study "only" with my friend （i never study alone or just classmate!)
ex) Mr.A 「普段映画見に行くの?」 Ms.B 「うん、彼氏"とは"行くよ」
Ms.B don't go to the movies except her boyfriend.
But you are not wrong. We use a lot of "とは" in Japan. Little bit harder to use it for non-native
But you can also continue the conversation as 「じゃあ、俺とは行かないの？」「いいえ、お前とも行くけど」
The とは just denotes a comparison between two things - in your case 彼氏 vs 友達 but we don't know whether other things also hold true or not.
We always say は is a "comparison" or "contrast" marker, along with the "topic" marker. We don't say it is a "limit" marker. "Only with my friend" is better written as 友達とだけ.
勉強します is the verb here "To study"
と is the particle "and/with" that marks "friend" as the person who you are studying with
友達と - with my friend 勉強します - study/do study
Reversing them would change the meaning and wouldn't make sense 勉強と友達します - [With a/the study][do friend]
Whether or not a を is needed depends on if the noun being done can also be a する verb. Not all nouns are able to be directly linked to suru (though in colloquial speech particles tend to drop anyway).
勉強 "study" is a suru verb, so you can either 勉強をする "do a study" or just 勉強する "study"
パーティー isn't a suru verb though, so you only パーティーをする "do a party"
There isn't a specific pattern to what is and is not a suru verb (other than logically some nouns you just can't turn into a verb). There are also some verbs that you may think would be a suru verb but aren't (釣り "fishing" is not a suru verb, so you can only 釣りをする "do fishing", but never directly 釣りする "fish")
Dictionaries will usually tell you if it can be used that way or not.
Noun, Suru verb
1. party (gathering, celebration, etc.)
Here's some links with more info on -suru verbs and how they function a bit differently
毎週日曜日 isn't an unreasonable way to answer it given how it's phrased in English. It's possible if relating a story in historic-present that "I study with my friend on Sunday" could refer to a single time you did so, but in 99% of cases it would imply it's an ongoing arrangement, i.e. "each/every Sunday", just as per "Sundays".
If you check the previous comments, a similar question has been asked and answered: https://forum.duolingo.com/comment/23250371?comment_id=49479417
勉強 is a so called "suru-verb" which means it is usually a noun but you can combine it with する (or its polite form します) without any particle in between to form a verb. Because of that, 勉強します in its entirety is the verb of this sentence (and not just します). Therefore it has to be put at the end of the sentence.
勉強します = "to study"
勉強をします = "to do studies" (which would often also be translated as "to study" since that sounds more natural)
If you use the particle を and と however, it can lead to confusions, because と also means "and". 友達と勉強をします sounds like "I do my friend(s) and studies.", therefore in that case, the other way around with 勉強 first, 勉強を友達とします, is preferred.
Both 勉強を友達とします and 友達と勉強します should be fine.
Not all nouns are suru-verbs. For example パーティー from the other questions is not. Which means パーティー always requires an を if used with します and hence "I party with friends" would generally be パーティーを友達とします, for the previously explained reasons.
Remember that particles always come after the word they are marking.
と being "and/with"
勉強と means "With studying/with a study" which wouldn't make any sense here
友達と is "with a friend"
友達 also is not a verb; you cannot 友達します 'do friend'
友達と勉強します "With a friend I study"
勉強と友達します "With a study I do friend"
To elaborate a little on what Keith said:
There is practically no difference in meaning between 勉強をします and 勉強します, but there is a grammatical difference:
勉強をします is the direct object 勉強 (study) and the verb します (to do)
勉強します is one verb as a whole (to study)
The difference becomes apparent when you want to modify them:
日本語を勉強します <-> 日本語の勉強をします
Both mean "I study japanese", but the first one uses 日本語 as the direct object for the verb "to study", while the second one uses 日本語の to modify the noun "study". Again, they mean the same thing, but use a different grammar structure. On the other hand: "日本語の勉強します" or "日本語を勉強をします" would both be grammatically wrong.
That grammatical difference is relevant for this sentence because the particle と can also be used to link two nouns, so you have to change the sentence order a little to make sure that you're still saying "I do it with my friend" and not "I do it and my friend":
友達と勉強します -> "I study with my friend."
友達と勉強をします -> "I do my friend and study."
勉強を友達とします -> "I study with my friend."