"It's not…" — how to negate is. One meaning, a whole ladder of politeness: じゃない with friends, ではありません in the exam room.
The short version
To say a noun (or な-adjective) is not something, drop だ／です and add the negative. The catch is that there are two independent dials: じゃ vs では is casual vs formal, and ない vs ありません is plain vs polite. Mix and match.
学生じゃない— casual "not a student"
学生ではない— written / emphatic
学生じゃありません ／ ではありません— polite → formal
Nouns and な-adjectives only. い-adjectives negate differently — 高い → 高くない, never 高いじゃない. See the adjectives page for that.
じゃない — the everyday form
By far the most common in conversation. It attaches straight to a noun or な-adjective.
🗣 じゃない? can flip to a tag question. With a rising tone it stops meaning "is not" and becomes "…right?" / "isn't it!" — 良いじゃない！ is "hey, that's good!", not "it's not good". Context and intonation tell them apart.
Two dials, not one. じゃ↔では chooses casual vs formal; ない↔ありません chooses plain vs polite. In fast speech you'll also hear the light polite じゃないです standing in for じゃありません.