Grammar Info
N5 Lesson 4: 13/13

くない
Not (adjective)

い-Adjectives, whether positive or negative, cannot be used with だ

Structure
Casual
Formal
[い]Adjective[く]+ ない

Examples:
おもしろ + ない
新し + ない
[い]Adjective[く]+ ない + です
[い]Adjective[く]+ ありません

Examples:
おもしろ + ない + です
おもしろ + ありません

新し + ない + です
新し + ありません
Details
Part of Speech Adjective
Word Type Auxiliary Verb
Register Standard
品詞 形容詞
単語の種類 助動詞
使用域 一般
Information
The negative form of い-Adjectives in Japanese is created by using the 連用形(れんようけい) form of the adjective, followed by ない. 連用形(れんようけい) form just means ‘agglutinative’, or ‘conjunctive’ in Japanese. For い-Adjectives, this conjunctive form is created by removing い, and replacing it with く.
  • あなた悪(わる)くない
    You are not to blame. (Translates directly to, you are not bad.)
  • 今日(きょう)暑(あつ)くない
    It is not hot today.
The ない used with い-Adjectives is considered to be a different word to the ない used with verbs. When used with い-Adjectives, ない itself is also an い-Adjective. This means that it follows the same rules as regular い-Adjectives. Due to ません (the polite negative form) being an auxiliary verb, it cannot be conjugated with い-Adjectives in the same way as verbs.
  • このお茶(ちゃ)、温(ぬる)くません。
    This tea don't lukewarm. (Incorrect example, with incorrect English to match!)
  • このお茶(ちゃ)、温(ぬる)くありません
    This tea is not lukewarm. (Natural Japanese)
As can be seen here, you must first add the verb ある, and then conjugate it into it’s negative form using the auxiliary verb ません. This is how the polite negative is created.
Caution
As with regular い-Adjectives, you cannot use after ない. です is acceptable to create a semi-polite form, but is best used in spoken language only.
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暑(あつ)ないです。[暑(あつ)い]
It is not hot.
[polite form]
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今日(きょう)寒(さむ)くない。[寒(さむ)い]
It is not cold today.
[short form]
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これ楽(たの)しないです。[楽(たの)しい]
This is not fun.
[polite form]
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Genki 1
Page 132