ではない

じゃない and ではない are the **negative forms** of the copula だ (to be). They are used to negate nouns and na-adjectives, expressing that something **is not** something. じゃない is the **casual/conversational** form, while ではない is the **formal/written** form. In meaning, they are identical.

Basic usage with nouns

Usage with na-adjectives

Na-adjectives also use じゃない / ではない for negation, since they behave like nouns grammatically.

Polite forms

The polite equivalents are じゃありません and ではありません. ではありません is more formal than じゃありません.

Past tense

To express "was not," use the past negative forms:

• Casual: じゃなかった / ではなかった
• Polite: じゃありませんでした / ではありませんでした

Confirmation and tag questions

In casual speech, じゃない with a rising intonation can also function as a tag question meaning "isn't it?" – similar to でしょう. Context and intonation distinguish this from the negative meaning.

Be careful not to confuse this confirming usage with the standard negative meaning.