"I did not study yesterday."
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About adding or not the は after 今日, 明日 etc, somebody please tell me if this is correct? I feel like it depends on the question/sentence that led to the one we're translating, for example: If someone asked "what did you do yesterday?" we would add the は because "yesterday" is the topic. If they asked "when did you study?" we would not add the は because "studying" is the topic. So both answers should be accepted because we don't have this context to base our answer on. How far from right is this?
I am no expert either but 今日 昨日 can definitely be used without the は. I am not very good with identifying the topic but I can assure you that without the は is correct. One question because you seem to know a bit... when answering a question, is the topic the information already stated on the question or is it the info you are being requested? For instance, "when did you study?" What you have stated is the studying part and you are requesting the moment in which the other person has done so. What woukd be the topic there?
しません is the polite negative version of する meaning "to do"。～ませ～ん alone is not a verb, it's just two auxiliaries that are attached to the i-stem of a given verb to transform it into a polite negative version.
In this case, the i-stem of the irregular する is し and then you attach ～ません to it. しません and しない are both the negative versions of します and する。
Other examples are:
ある → あります → ありません
いる → います → いません
話す → 話します → 話しません
飲む → 飲みます → 飲みません
In this sentence you are also using a past negative form of する, しませんでした is whole. I know is confusing because the past of です is also でした、but it's better if you see しませんでした as one word than a combination of different things. It gets easier later on.
Using the を is also technically correct. べんきょうします (benkyoushimasu) is somewhat special in that it doesn't need an を though. べんきょうをします would basically be like saying, "I do studying." It's not wrong, but it's unnecessary when you could just say "studying" to mean the same thing.
For example, if someone asked what you were doing tonight, you'd normally just answer, "I will be studying," not "I will be doing studying."
TLDR: It's neither in this case; technically を is the correct particle, but it's not used either because 勉強 is a suru noun.
Long Answer: は is already marking 昨日 as the topic, contextualizing the sentence similar to "as for yesterday". As far as I know, there can only be one topic per sentence, and so only one topic marker. You can have は mark 勉強 if you want but I think you have to remove it from 昨日 first.
が is the subject particle, it marks whatever noun is executing the verb in the sentence. The verb in this case is "しませんでした"/"to not have done", dreived from する/"to do", so が is marking the (not) doer. I'm not sure there are a lot of examples of 勉強 (literally "the act or process of studying") as a subject noun, so 勉強が may be a rare phrase. The example you may have come across must have been " 勉強が好きです。" meaning "I like studying" but is syntatically structured more like "Studying is pleasing." The distinction in structure is important: が is used here because 好き is an adjective and です is the closest thing Japanese has to the English verb "to be".
勉強 in this sentence however isn't the subject; in fact " 昨日は勉強（を）しませんでした。" doesn't have an explicit subject; The English translation tells us that it's supposed to be "me/I", which means there is an implied "私が" hidden in the message. Instead 勉強 is a direct object, the noun that is acted upon by the verb, in this case the thing that is being done. This is why I inserted the object marker を after it here.
Now are you ready for a twist? I lied about 勉強 being a direct object in this sentence. The truth is that it part of the verb 勉強する (or 勉強します for politeness). How does this work? Well, 勉強 is what we call a suru noun, a noun that can be turned into a verb if you add the base verb する to the end of it. That's why を is left out. The difference is basically like using the word "study" as a verb vs. as a noun in English:
私が勉強をします = I do the studying.
私が勉強します (no を) = I study.
This is pretty complicated to answer but I'll try. でした by itself is the just the past-tense form of です, like "was" is the past-tense form of "is". Same with ～ました being the past tense form of ～ます. The difference is that the です and でした can exist on their own because they're the Copula Verb, but ～ます and ～ました are suffixes that must be attached as verb endings. For example, let's use the suffix on 食べる/"to eat". 食べます meaning "I eat" can turn into 食べました meaning "I ate"/"I have eaten".
Here's where it gets complicated: to get the negative version of a past-tense verb, you must first negate the present-tense form, usually by replacing ～ます with ～ません, AND THEN YOU ADD でした right after it to make it past-tense. This means turning 食べます(I eat) into --> 食べません(I don't eat) into --> 食べませんでした, all just to say "I did not eat" or "I have not eaten". Also to say "was not" you'd say: ではありませんでした. That is two different forms of です joined together right? What a mess!
To make it easier I'll link these tables real quick: http://www.japaneseverbconjugator.com/BeingVerbs.asp
duo marked it wrong im just wondering, does it make any sense to use the "ga" particle here like 昨日は勉強がしませんでした from what limited knowledge I have this should just mean like "yesterday, WORK isn't what I did" (but like... I did so SOMETHING yesterday. it just wasn't work). does this make any sense or just straight up wrong?
nnnnn-No... を would be the correct particle to use here, not が. The difference between subject (marked by が in Japanese) and direct object (marked by を) is that the subject executes the action of the associated verb, whereas the object is the thing that the verb is executed upon.
If you used が the way that you did, then the English translation would come out something like "Yesterday, Studying didn't do." Nevermind that "studying" isn't often a grammatical subject (I mean what kinds of things can the proccess of studying itself even do?), more importantly, the verb します/"to do" is transitive in both languages, meaning it requires an object to work properly. The "studying" needs to be doing something, it can't just do.
Side note: 勉強 is what's called a "suru noun", so named because you can add any conjugation of する — this incudes しませんでした — to the end of it to turn it into a verb- without the need for a particle, which is why Duolingo's sentence here doesn't have one. (Also, I'm assuming that you just got confused about which verbs we were practicing in these lessons. It happens, but reminder: 仕事 is "Work"; 勉強 is "Studying".)
Also also, judging from the implicit meaning you were trying to convey in your example, I assume this one of those は-vs-が questions? It's best not to think of は as a rigidly grammatical particle like が or を, because that would imply that its meaning as a topic marker is somehow seperate from them; instead, you should think in terms of which other particle it's supposed to replace or substitute. That's what's really going on with は: a noun that already has its own grammatical purpose is being emphasized as the topic of the sentence.
でした is what makes this sentence past-tense
します do / will do (habitual/future)
しません do not do / will not do (habitual/future negative)
しました did (past)
しませんでした did not do (past negative)
～ます is a polite non-past verb ending
います is the ～ます form of the verb いる "exist (animate)"
Duolingo occasionally splits up words spelled in hiragana (when you have options to click on words rather than type them). So I can't tell if something is one word or two. Is でした (deshita) one word or two? Or a a suffix rather than a stand-alone word? The same goes for arimasu and imasu; are they one word each or two?
します is the polite form of the verb する "to do"
勉強 is the noun "a study" so it combines with the verb する "do" to create the phrase 勉強する "to study" or in polite form 勉強します
～ます and the other polite inflections cannot attach to 勉強 directly as it is a noun, not a verb.