じゃない

About

Often translated as "not" or "is not", じゃない is technically a combination of the copula 「じゃ」and the い-adjective 「ない」. The じゃ copula itself is an informal contraction of 「では」.

じゃない attaches to な-adjectives and nouns, and makes them negative. じゃない is a casual negator, and is therefore not suitable on its own for polite or formal situations. However, じゃない can be made more polite by conjugating into its ます form (じゃありません) or by adding です. Of these two, the 「じゃないです」pattern is more common.

Since the final ない of じゃない is an い-adjective, we can conjugate it into the past tense by turning it into じゃなかった. This can be made more polite by adding です.

じゃない can also be used to ask questions, which is usually indicated by rising intonation. When used to ask a question, じゃない is often translated as "Isn't (it)?" (e.g. 「テストは明日あしたじゃない？」"Isn't the test tomorrow?").

Summary

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  じゃない makes a noun or な-adjective negative.

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  じゃない is casual.

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  The past tense of じゃない is じゃなかった.

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  じゃない can be made polite by adding です, or by conjugating to じゃありません (じゃないです is more common).

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  じゃない can be used to ask "isn't it?" type questions

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