んじゃない

んじゃない after a verb in the dictionary form is a spoken "don't!": a reprimand, the way a parent tells off a child or a teacher a pupil. Said with a falling tone, it is an order, not a question.

It comes from someone with the standing to scold. Said to a superior or a stranger it is rude; ask with ないでください instead. The bare prohibitive な – なく泣くな – is blunter and rougher than んじゃない.

Regret: んじゃなかった

In the past, aimed at yourself, it means "I shouldn't have": you did it and wish you hadn't.

The verb stays in the dictionary form; only ない changes to なかった.

• × 食べたんじゃなかった → 食べるんじゃなかった

Rising tone: a guess

With a rising tone and a question mark, the same words are a guess: "I think ~, don't you?" This is a different pattern. It takes any plain form, not only a verb.

くる来るんじゃないきごう！ with a falling tone is "Don't come!"; くる来るんじゃないきごう？ with a rising one is "I think he's coming".