〜くない
isn't (い-adjective)
い-adjective − い + くない (polite: くないです / くありません)
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To make an い-adjective negative, drop the final い and add くない: 高い → 高くない, "not expensive"; 面白い → 面白くない, "not interesting".
Politely, add です (高くないです) or use the ません form (高くありません). Both are accepted here; くないです is the more common in conversation.
The trap is reaching for じゃない, which belongs to nouns and な-adjectives: 高いじゃない is wrong.
いい, "good", is the one exception: its forms are built on an older word, よい. So "not good" is よくない, never いくない.
Examples
この店はあまり高くない。
This shop isn't very expensive.
今日は寒くないです。
It isn't cold today.
この問題は難しくないよ。
This question isn't hard.
天気がよくないですね。
The weather isn't great, is it?
駅はここから遠くないです。
The station isn't far from here.