んじゃない

The expression んじゃない is a colloquial prohibition form used to strongly tell someone not to do something. It is equivalent to "Don't ~!" or "You mustn't ~!" and carries a scolding or commanding tone. It is formed by attaching んじゃない to the dictionary form of a verb.

Prohibition – "Don't ~!"

This usage has a falling intonation and expresses a firm command not to do something. It is used by parents scolding children, or among close friends and family in casual settings.

This form is quite direct and strong. It would sound rude if used toward superiors or strangers. For polite situations, use ～ないでください instead.

Regret – "Shouldn't have ~"

The past form んじゃなかった expresses regret about something one did. It translates as "I shouldn't have ~."

This is a very natural way to express personal regret in casual conversation.

Watch out: Conjecture usage

With a rising intonation, んじゃない？ can instead mean "Isn't it ~?" or "I think ~", expressing a guess or seeking confirmation. Context and intonation are key to distinguishing the two:

Compare: くる来るんじゃないきごう！ (falling ↓) = "Don't come!" vs. くる来るんじゃないきごう？ (rising ↑) = "Isn't he coming?"