〜ない

1. 1Plain negative of a verb: does not do, or will not do.Ex. 1
2. 2Plain negative of an adjective or noun predicate: is not.Ex. 2

This is the plain-style negative, used in casual conversation with friends and family and in plain-style writing such as diaries and essays. In polite speech, verbs use 〜ません, い-adjectives use 〜くないです or 〜くありません, and な-adjectives and nouns use ではありません or じゃないです. It often pairs with adverbs such as あまり to soften the negation ("not very"), as in example 2.

• 〜なかったN5「〜なかった」 is the past tense of 「〜ない」, used to negate past actions or states (didn't go yesterday, didn't look good back then). Note that 'haven't eaten yet' should be 「まだ食べていない」, not 「まだ食べなかった」.
• ないでN3「〜ない」 is the negative form; it attaches to verbs, adjectives, and nouns, and can end a sentence on its own. 「ないで」 attaches only to verbs and links to a following clause meaning 'without doing...' or 'not... but...', though at the end of a sentence it becomes a request meaning 'don't...'. The two are not interchangeable.
• ずにN3「ずに」 attaches only to verbs and mainly connects two actions within a sentence, meaning 'without doing...' or 'not... but...'. It generally does not stand alone as a predicate and does not attach to adjectives or nouns. It is formed by replacing 「ない」 with 「ずに」, with する becoming 「せずに」, and is somewhat formal/written-style.
• ないでくださいN5「〜ない」 simply states a negation and can attach to verbs, adjectives, and nouns. 「ないでください」 is built from a verb's ない-form plus 「でください」 and is used to politely ask someone not to do something; it cannot be swapped with 「〜ない」.
• なくてもいいN4「〜ない」 only states 'not do / not be'. 「なくてもいい」, built on the ない-form, means 'it's fine not to do / it's okay even if it's not', used to waive an obligation or give permission. The two differ in meaning and are not interchangeable.
• なければいけない / なければならないN4「〜ない」 only expresses negation 'not / didn't'. 「なければいけない／ならない」 is formed by dropping the い of the ない-form, adding 「ければ」, then attaching 「いけない／ならない」, and means 'must do...'. The two are not interchangeable.

〜ない is the basic present or future negative, while 〜なかった is its past form, ないで and ずに mean "without doing" (ずに is more literary), and ないでください asks someone not to do something. なくてもいい ("don't have to") and なければいけない / なければならない ("must") are built on the ない form but express permission and obligation rather than simple negation.