～た

～た (or its voiced twin ～だ) is the plain past tense ending. It attaches to verbs (食べた = ate, 読んだ = read) and to i-adjectives (高かった = was expensive), covering English “did”, “ate”, “was ...”.

The same た also hides inside polite ～ました and でした. There, past and polite are two separate ideas stacked together: 食べました = polite ます + past た.

1. Completed action

Use ～た on a verb to say that an action happened and is finished.

2. Past state or description

On an i-adjective, the ending becomes ～かった and describes how something was.

How to form it

For godan (u-verbs), the final syllable changes sound before た: う・つ・る → った, む・ぶ・ぬ → んだ, く → いた, ぐ → いだ, す → した; the one exception is 行く → 行った. Ichidan (ru-verbs) drop る and add た. Irregulars: する → した, 来る (くる) → 来た (きた). I-adjectives replace い with かった, and negative ない becomes なかった.

• 買う（かう）= to buy → 買った（かった）= bought
• 読む（よむ）= to read → 読んだ（よんだ）= read
• 書く（かく）= to write → 書いた（かいた）= wrote
• 泳ぐ（およぐ）= to swim → 泳いだ（およいだ）= swam
• 話す（はなす）= to speak → 話した（はなした）= spoke
• 行く（いく）= to go → 行った（いった）= went (exception)
• 食べる（たべる）= to eat → 食べた（たべた）= ate
• する = to do → した = did
• 来る（くる）= to come → 来た（きた）= came
• 高い（たかい）= expensive → 高かった（たかかった）= was expensive
• ない = not → なかった = was not / did not

Key idea

～た marks something as done and settled: the action is complete, or the state held true in the past. Politeness and negation are separate pieces that stack around it.