N3
んじゃない
Isn't it…? (gentle guess or check); Don't…! (casual prohibition)
いつまでもケラケラしながら話してんじゃない！
Don't keep laughing while you're talking!
Explanation
「んじゃない」 has 3 uses
1Isn't it…?
With rising tone, gently gives your view or asks for agreement: 「これ、おいしいんじゃない？」
本当は彼も来たかったんじゃない？
Really though, didn't he want to come too?
山本さんなら、もう帰ったんじゃない？
Yamamoto? Hasn't she gone home already?
2Don't!
With falling tone after a dictionary-form verb, a sharp 'don't', often from parent to child: 「走るんじゃない！」
僕をバカにするんじゃない！
Don't make a fool of me!
いつまでもケラケラしながら話してんじゃない！
Don't keep laughing while you're talking!
3Shouldn't have
「〜んじゃなかった」: regret for something you did, 'I shouldn't have…': 「あんなこと言うんじゃなかった」.
大学を中退するんじゃなかった…。
I shouldn't have dropped out of university…
事故を起こした…スピードを出すんじゃなかった。
I've had an accident… I shouldn't have been speeding.
Mixed up with
The tone is often tested in quick-response listening. Rising 「～んじゃない？」: answer by agreeing or giving your view. Falling 「～んじゃない！」: a strict prohibition and scolding.
For the guessing use, add 「な」 after nouns and na-adjectives: 「病気なんじゃない？」, 「暇なんじゃない？」. A bare noun is wrong.