if you've been listening to people speak japanese casually, you've probably heard these conversational sentence-final particles! here i'll go over the difference between the two and then give their usages and example sentences. 行こう！
wait, doesn't じゃない just mean "not"?
you would be right about that! but as with lots of things in spoken language, intonation is key here. compare these two sentences in english:
it isn't pretty. (falling intonation)
isn't it pretty? (rising intonation)
clearly, these two sentences mean different things. they use different intonation, but even more obviously, the words come in different orders—in european languages, that's one of the easiest ways to tell a statement from a question. but as we know, in japanese, questions are made without moving words around; so, intonation alone with have to do the job! now, compare these two:
きれいじゃない。 (falling intonation) = it isn't pretty.
きれいじゃない？ (rising intonation) = isn't it pretty? / it's pretty, don't you think?
thanks to the rising intonation in the second sentence, instead of negation, we get a tag question. so if you're reading instead of listening, watch out for situations where you might have to guess or intuit what the intonation of the sentence is!
the difference between 〜じゃん and 〜じゃない
in terms of their conversational meaning or usage, these two particles are the same. however, in terms of "indexing" and social cues, じゃん is a bit more casual and じゃない can have a feminine tone. compare these sentences:
きれいじゃん？ (casual)
きれいじゃない？ (casual, feminine)
きれいじゃないですか？ (a bit more polite)
these sentences all feel very conversational, but depending on how you want to present yourself you might pick one over the others.
also, it's worth noting that じゃん only has the conversational usage: no matter the intonation, it can't mean "not" in regular statements (as far as i've seen, anyway!).
what's the difference between 〜じゃん／じゃない and 〜ね?
both these sentence-final particles ask for agreement or confirmation, as in these two sentences:
きれいじゃん？
きれいだね。
both of these could be translated as "isn't it pretty?" or similar. however, the tag question denoted by 〜じゃん is stronger than the tag question denoted by 〜ね. in other words, ending with じゃん is more like an actual question than ending with ね.
for example, if you said きれいじゃん？, you might be expecting your conversation partner to have had some expectation subverted. maybe you're looking at modern art, and they had initially expressed a disinterest in modern art; saying きれいじゃん？ leaves more room for them to change their opinion or even to disagree with you.
on the other hand, きれいだね is tantamount to a gentle statement, like you already know your conversation partner will agree with you. you might say きれいだね while overlooking a 絶景 (ぜっけい = superb view) such as 富士山 (ふじさん = mt. fuji) or 琵琶湖 (びわこ = lake biwa). after all, who could ever not find those きれい!
addendum: 〜じゃん with falling intonation
i watch someone on youtube who makes lots of designs in animal crossing, and when she is satisfied with a layout she often says 「いいじゃん！」, meaning "isn't that great!" or similar. but why is she using じゃん if she isn't actually asking for confirmation?
this seems to be an instance of a rhetorical tag question. in particular, the intonation of her いいじゃんs is falling, so it doesn't sound like a question. instead, she's making an exclamation out of a question. this is similar to what we might do in english with an exclamation like "isn't that great!", where we might pitch the sentence as a statement instead of a question. (and, to be totally clear, this person also says 「いいんじゃない？」 a lot too!)
例文 (れいぶん = example sentences)
高（たか）すぎじゃない？ = don't you think that's a bit expensive?
この辺（へん）は神社（じんじゃ）が多い（おおい）じゃない？ = aren't there a lot of shrines around here?
疲れて（つかれて）いるんじゃない？ = aren't you tired? (don't you just want to go apeshit?)
全然（ぜんぜん）辛く（からく）ないじゃん？ = it's not spicy at all, right?
やればできるじゃん！ = you can do it if you try!
だから言った（いった）じゃん！ = it's like i already told you!
summary
in summary, 〜じゃん／じゃない are casual conversational particles you can use to make a tag question like "isn't it?" or "don't you think?", and they tend to have a rising intonation. of course, there are complications and exceptions to everything in language, so keep your eyes peeled and you'll definitely see more interesting usages of these particles!
as always, feel free to send me an ask if you have any questions/thoughts. じゃねー！
main sources:
I'd like to know if I can put ～じゃん at the end of every adjective, if there are any exceptions to that usage, and if it's different from ～じゃな
I noticed in an anime I watched, one of the characters said something like below:
さっき食べたじゃない。
And what I think the meaning is: Didn't
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