I’m going though the じゃなかった/was not section over on bunpro, and I’m a little confused about something. I know particles can be dropped fairly arbitrarily in casual speech, but the lack of particle in their example sentences is confusing me. For example: I would expect 先生じゃなかった to be 先生はじゃなかった、 at least in the written language.
Basically, are they being overly casual and would one typically expect a particle here, or is this a particle free scenario even in more formal scenarios? Thanks!
じゃない is the negative form of です. 先生じゃない is just like saying “is not teacher,” where the subject is implied (such as わたしは先生じゃない）
also, it is causal, the more formal would be じゃありません
The degree of formality goes like this:
じゃありません
じゃないです
じゃない
I don’t think your second version makes much sense, though I could be wrong. Either way, I think you’re getting muddled, because you wouldn’t have は after 先生 to begin with (there might be something I’m not thinking of here).
先生じゃなかった means “wasn’t a teacher” - whatever the subject of conversation (presumably implied), they weren’t a teacher
先生はじゃなかった would mean “as for the teacher, they weren’t”
A more full sentence might be: 父は、先生じゃなかった
“My dad wasn’t a teacher”
You could drop the particle to make 父、先生じゃなｋった
Here, you’ve dropped the particle indicating the subject.
Oops! Yeah, I’ve been learning a lot of new grammar this week and I’m definitely getting a few things conflated. Nounです totally makes sense, as would the negative form of that. In retrospect I’m not even sure how I got confused here. Thanks all!
じゃ is a contraction of では, so actually the particle is still there. 先生ではない=先生じゃない
I like to think of ない as the verb, and じゃ as a contraction and it makes sense for me.
I’m still learning too so if there’s anything wrong with what I said please tell me.
I agree that じゃありません is more formal than じゃないです, but is it really right to call “じゃないです” “casual”? We are still using the copula, and it’s still distal speech.
I think it’s because learning an entirely different language is confusing 
ない is a verb - it’s the plain negative form of ある. 
Well あらない does exist.
Also, the most formal would be: 先生ではありません
Right?
Well you’ve also got ござる.
Aye, and let’s not even get into keigo. 
I don’t think ござる makes you sound more formal, just a weirdo. ございません on the other hand, that’s fine.
I was simply giving the dictionary form. But yes, that would indeed make you sound like a weirdo.
(Good catch though, I might have very well been confusing)
Since じゃ is a contraction for the particle pair では, there is no particle missing. The contraction is because では is almost always shortened to じゃ in spoken language, even in polite language (じゃありません). In honorific language you’d use でございません which noticeably doesn’t include は, so there’s no possibility for contraction.
You’re not totally off the mark in that really informal language can be missing particles, and also that this is an informal sentence (polite langue would throw an んです at the end, or convert the verb into long form as in my parenthetical above) but in this case it’s just that sentence written here is (presumably) a sentence of dialogue rather than literary narration so the contraction is being used.
I think it’s more semantics than anything, but I don’t think @rmizuno necessarily meant all forms using じゃない are casual, since the first post mentioned only じゃない, the the second indicating the forms that increase formality.
The problem with ないです is that it’s function isn’t strictly within alignment to the simple ない and ます. It projects distality but isn’t used to make the viewer more polite, so the semantics behind formality, distality, and politeness become a sort of mess. For purposes of better meaning, I’m comparing Formal to Informal (Formality), Familiar to Distal (Distality), Casual to Polite (Politeness)
じゃない: Informal, Familiar, Casual
ではない: Informal, Familiar (Forceful), Casual
じゃないです: Informal, Distal, Casual (Semi-Polite)
ではないです: Informal, Distal, Casual (Semi-Polite)
じゃありません: Formal, Familiar, Polite
ではありません: Formal, Distal, Polite
The usage of ない takes the formality out of the statement, while the です adds distance between the participants, at least for the topic of the conversation.
Semantics is what’s being discussed in this thread though! Thanks for a wonderfully detailed post.
Cough, cough. では.
In any case, I think you’ll also generally find that it’s usually じゃ in speaking and では in writing, regardless of formality level, unless you’re being extremely formal.
Woops, thanks for that. I have not yet gotten to the stage of enlightenment where I type straight kana, so I sometimes mess up and type phonetically. 
Yup, じゃ would only appear in writing during dialogue. I have heard both quite commonly in speaking though, interestingly enough. And, of course, in anime, there is a whole spectrum of verbal nonsense you’ll never hear anywhere else.