〜ない Inflects Like an i-Adjective

This is the single most powerful idea in Japanese verb negation, and most learners meet it by accident, one confusing form at a time. Stated once, clearly, it dissolves a dozen "irregular" negatives: 〜ない is not a frozen ending — it is a real い-adjective. It ends in い, it means something adjective-like (non-existent / not-the-case), and it inflects for tense and connects to other clauses using the exact rules of 高い（たかい）. Once you see that, the past-negative, the negative te-form, and "not very" all stop being separate facts and become one fact you already knew from い-adjectives.

The parallel, in one table

Put the plain negative 食べない next to the ordinary い-adjective 高い. They march in lockstep — every column is the same operation on the final い.

The recipe is identical in every row: drop the final い, then attach the adjective ending — かった for past, くて for te, く for adverbial. There is nothing verb-specific happening. ない just is an い-adjective wearing a negative meaning.

Past negative: 〜なかった (never ×〜ないだった)

This is where the insight pays for itself. To make a plain negative past, you do not stick a copula-past on the end. You inflect ない itself, exactly like 高い → 高かった: drop い, add かった.

The reason 来なかった is right and ×来ないだった is wrong is the whole thesis of this page: ない is an adjective, and adjectives take かった for the past — they never take the copula past だった. So the negative past unifies with adjective inflection instead of standing off as a special verb rule. (This form is common enough to get its own page; here the point is why it looks the way it does.)

Negative te-form: 〜なくて for reasons and links

The te-form of an い-adjective is 〜くて (高くて, (it's) expensive and...). Apply that to ない and you get 〜なくて, which links a negative clause to what follows — very often as a reason (because ... not).

Both なくて forms come straight from ない → なくて, the same shape as 高い → 高くて. And note 行けなかった in the second sentence: the potential 行ける negates to 行けない, which — being another い-adjective in disguise — pasts to 行けなかった. The pattern compounds cleanly.

"Even if not": 〜なくても and あまり…ない

Because なくて is a real te-form, it takes も to build 〜なくても (even if ... not), just as 高くても means even if it's expensive.

And the adverb あまり ((not) very / not much) leans on the plain ない to mean "not much," an everyday partnership:

The adverbial 〜なく (formal / written)

The bare adverbial form — 高い → 高く, so ない → なく — is the one cell you meet less often in casual speech, because everyday Japanese usually reaches for the te-linked 〜なくて instead. But 〜なく is alive and well in formal and written Japanese, and it is the hidden engine behind a whole shelf of common set adverbs.

問題ない (there's no problem) and 間違いない (there's no mistake) are ない-phrases; drop the い and add く and you get the adverbs 問題なく (without a hitch) and 間違いなく (undoubtedly). Once you see that these are just the adverbial of ない, they stop being vocabulary to memorize and become forms you can predict. (formal / written)

Politeness: 〜ないです ≈ 〜ません

Because ない is an い-adjective, you can make it polite the adjective way — by adding です, just like 高いです. So 食べないです is a valid polite negative, roughly equal to 食べません.

Both 行かないです and 行きません are correct and polite. The ません form is a touch more formal and is what textbooks drill first; the 〜ないです form is extremely common in real speech, especially when the rest of the sentence was already in plain form and you tack です on to round it off politely. The past works the same way: 行かなかったです sits alongside 行きませんでした.

Common mistakes

Every mistake above is the same mistake: treating ない as an invariant particle and gluing tense on the outside. Treat it as the い-adjective it is — drop the い, attach かった / くて / く — and the errors vanish.

Key takeaways

• 〜ない is a genuine い-adjective; it inflects like 高い.
• Past: drop い, add かった — 食べなかった, 来なかった. Never ×〜ないだった.
• Te-form / reason: 〜なくて — お金がなくて困った.
• Even if not: 〜なくても; not much: あまり…ない.
• Polite: 〜ないです ≈ 〜ません; past 〜なかったです ≈ 〜ませんでした.