んじゃない (日本語能力試験 N3)

「んじゃない」の情報

んじゃない is a slightly slang grammar pattern used with verbs to prohibit or request that something not be done. It will usually just be translated as 'don't do (A)', or 'you mustn't (A)'. んじゃない is almost always used from a higher position toward someone of a lower status. It may be used when the speaker is angry, but not always.
んじゃない is a combination of the case-marking particle の, and the abbreviated form of ではない, じゃない. It will connect to the dictionary form of verbs.

親(おや)に向(む)かってそんなこと言(い)うんじゃない。
Don't say something like that directed at your parents!
そこら辺(へん)のキノコを食(た)べるんじゃない！
Don't eat the mushrooms from around there!
今日(きょう)は混(こ)んでるからパパから離(はな)れるんじゃないぞ！
It's crowded today, so don't get separated from your dad!

Despite being slang, the slightly more formal version んじゃありません can also be seen occasionally. This variation is common when the speaker wants to forbid something, but not sound angry, such as when an adult is talking to a child.

勝手(かって)にお友達(ともだち)のものを取(と)るんじゃありません。
You shouldn't just freely take your friend's things.
乳児(にゅうじ)にハチミツをあげるんじゃありません。
Don't give honey to infants.

The more literal translation of Verb + んじゃない is just 'it's not (verb)'. This sounds a bit strange in English, but might be similar to something like 'you're not (verb)', where a person is directly telling another what they shouldn't do.
It is also common to see the abbreviated ている form, てん, before じゃない.

いつまでもケラケラしながら話(はな)してんじゃない！
Don't keep laughing while you're talking!
一日中(いちにちじゅう)ダラダラしてんじゃない！いい加減(かげん)、仕事(しごと)でも探(さが)してこい！
Don't laze around all day! Get your act together and find a job!

Caution - When used in the past tense, んじゃなかった indicates that something 'should not have been done', and has a nuance of regret or disappointment that the action occurred.

こんな安(やす)い車(くるま)を買(か)うんじゃなかった。
I shouldn't have bought a cheap car like this.
あんな怪(あや)しい話(はなし)に乗(の)るんじゃなかった。
I shouldn't have gotten swept up in such a suspicious story.

Caution - When んじゃない comes at the end of a sentence, it will be important to listen to the intonation, as the statement may actually be a question. In this case, it is an abbreviation of んじゃないか.

本当(ほんとう)は彼(かれ)も来(き)たかったんじゃない？
Really though, didn't he want to come too?

The difference between these two uses of んじゃない will usually be very clear. A question has a rising intonation, while an order will have a dropping intonation.