Grammar JLPT Level N5 - Lesson 12

Structure of じゃない:

［な］Adjective + では⁽¹⁾ + ない
Noun + では⁽¹⁾ + ない

⁽¹⁾じゃ

Details about じゃない:

じゃない is the casual form of ではない. では is regularly contracted to じゃ in Japanese, so is worth remembering as its own structure. じゃない (casual), or ではない (semi-polite) are the structures used for creating the negative form of nouns and な-Adjectives.
These structures are simply added to the base form of the noun or な-Adjective.

これは食(た)べ物(もの)じゃない。
This isn't food.
この携帯(けいたい)は便利(べんり)じゃない。
This cell phone is not useful.

じゃ and では can also be used with the negative ます form of ある, to create the polite variant.

それは飲(の)み物(もの)ではありません。
That is not a drink.
あなたの部屋(へや)は綺麗(きれい)ではありません。
Your room is not clean.

ない is sometimes said/written as ね or ねえ. However, this is considered very casual, or even a bit rough. Accordingly, it is best avoided in most conversations.

俺(おれ)は馬鹿(ばか)じゃねえよ。
I'm not stupid.

じゃない

Usage Notes for じゃない:

じゃない is used very frequently to ask questions. This is similar to じゃないか, a grammar point covered in another lesson. However, the nuance difference is that じゃない as a question is a little bit more blunt, like 'isn't it?' in English. On the other hand, じゃないか is more like 'don't you think?'.