んじゃない

んじゃない is a casual, emphatic way to tell someone not to do something or to admonish against a course of action. It often functions as a prohibition or strong advice, similar to “Don’t …” or “You shouldn’t …” in English. It can also be used to deny or challenge a situation when appended to nouns or adjectives, though its primary usage is prohibitive. The tone is rougher and more masculine/colloquial, and it’s commonly heard in spoken Japanese among friends, family, or in admonishing contexts. It often appears after the plain form of verbs, and can attach to i-adjectives. When used with a question-like intonation, it can express mild surprise or rhetorical expectation rather than a strict command.