This means 'is not'. You use it to say something is not a noun. You also use it to say something is not a 'na'-adjective.
Noun + じゃありませんPrimary
な-Adjective + じゃありません
Noun + じゃありませんでした
な-Adjective + じゃありませんでした
This means 'is not'. You use it to say something is not a noun. You also use it to say something is not a 'na'-adjective.
すみません、私は田中じゃありません。
Excuse me, I am not Tanaka.
この辺りは夜、安全じゃありません。
This area is not safe at night.
子供の時、ここは公園じゃありませんでした。
When I was a child, this place was not a park.
彼の説明は十分じゃありませんでしたが、何とか理解できました。
His explanation wasn't sufficient, but I managed to understand somehow.
警告：バッテリー残量は十分じゃありません。
Warning: The battery level is not sufficient.
This is different from "dewa arimasen" because "dewa arimasen" is more formal. You use "dewa arimasen" when you write. You also use it in formal talks. You use "ja arimasen" in daily polite talks. The very casual way to say it is "ja nai".
Watch out: You cannot use this with "i-adjectives." For example, don't say "atarashii ja arimasen." That is wrong. Say "atarashiku arimasen" instead.
Don't use this when you want to say an "i-adjective" is not something. For example, don't say "Takai ja arimasen" for "It is not expensive."
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