Plain Past-Negative 〜なかった

The plain past-negative — the 〜なかった form — is how you say "didn't do" or "hasn't done" when you are speaking casually: with friends, family, in a diary, or inside your own head. It is the casual counterpart of the polite 〜ませんでした. And here is the good news up front: there is nothing new to memorize. If you can already make the plain negative 〜ない, you already have the past-negative — you just inflect that ない the same way you inflect any i-adjective.

The rule: ない → なかった

Take the plain negative form and change its final ない to なかった. That's the entire rule, and it works for every verb class without a single exception.

Why does it work so cleanly? Because the negative ending ない is not a verb — it behaves exactly like an i-adjective. And the past of any i-adjective is made by dropping the final い and adding かった: 高い → 高かった (takakatta, "was expensive"), 楽しい → 楽しかった (tanoshikatta, "was fun"). Apply that same move to ない and you get なかった. The past tense lives inside かった, not anywhere else.

The two irregular verbs stay simple

Japanese has only two genuinely irregular verbs, する and 来る, and both are painless here because the irregularity has already happened by the time you reach the negative. する → しない → しなかった. 来る → 来ない (こない) → 来なかった (こなかった). Just note the reading: 来なかった is konakatta, not kinakatta — 来 shifts to the こ-stem in the negative, exactly as it does in 来ない.

Where English speakers go wrong: ない already carries the past

This is the one mistake almost every English speaker makes at least once. In English, "did not eat" needs a past-tense helper — did. So learners instinctively hunt for a past-tense word to bolt onto Japanese too, and they reach for the copula's past だった or でした:

The pastness is already inside なかった — the かった is the past. Adding だった or でした on top is like saying "didn't didn't." Once you see ない as an i-adjective rather than an English "not," the temptation disappears: you would never say ×高いだった for "was expensive," and 食べないだった is the exact same error.

The polite version: two correct shapes

Because なかった is literally the past tense of the adjective ない, it produces something learners often find surprising: the plain past-negative has two legitimate polite forms, and they mirror the copula precisely.

The first route rebuilds the sentence around ます and gives you 〜ませんでした (formal / neutral-polite). The second route simply adds です to the plain 〜なかった, giving 〜なかったです (informal-polite / conversational) — polite because of です, but built on the casual stem. Both are correct.

This is the same fork you see in the copula's past-negative: the formal ではありませんでした versus the everyday じゃなかったです. In both cases modern conversation drifts steadily toward the lighter, なかったです-shaped form, while the ます-based form stays as the safe choice for genuinely formal settings. The practical takeaway: recognize both, but you only ever need to produce one.

That last example is a real i-adjective (高い), and it takes exactly the same かった ending as なかった — proof that ない is just another adjective in disguise. Learn the i-adjective past and the verb past-negative for free, in one stroke.

Register: this is the casual form

〜なかった is casual register. Use it with people you are close to, in notes and texts, and — importantly — inside subordinate clauses even within polite speech, because the plain form is the default anywhere that isn't the final verb of a polite sentence:

Notice how しなかった sits casually inside the から-clause while the sentence still ends politely in 〜ました. To end a sentence politely with a stranger or superior, switch to 〜ませんでした.

Common mistakes

Key takeaways

• One move: take the 〜ない form and change ない → なかった (the i-adjective past, い → かった).
• The past lives in かった. Never add だった or でした on top — that double-marks the tense.
• The irregulars behave: しない → しなかった, 来ない → 来なかった (read konakatta).
• Polite has two shapes: the formal 〜ませんでした and the modern 〜なかったです — recognize both, produce one.