Past tenses

Japanese has two tenses, not three: past and non-past. The non-past covers present and future; the past is one suffix, -ta (plain) or -mashita (polite). What makes the past worth its own chapter is that it is the one place where the conjugation machine produces sound changes.

The easy half

Vowel stems just take the suffix, as always:

tabe + (i)mashita  →  tabemashita   食べました
tabe + ta          →  tabeta        食べた
mi   + ta          →  mita          見た

Polite past is always regular, for every verb in the language: -mashita. If you are ever unsure, the polite form is the safe road.

The messy half: 音便 (euphonic change)

Consonant stems collapse when -ta lands on them. The consonant and the t fuse:

stem ends in	becomes	example
く k	いた ita	書く kaku → 書いた kaita
ぐ g	いだ ida	泳ぐ oyogu → 泳いだ oyoida
む m / ぶ b / ぬ n	んだ nda	飲む nomu → 飲んだ nonda
る r / つ ts / う (w)	った tta	買う kau → 買った katta
す s	した shita	話す hanasu → 話した hanashita

This is the system's only real ugliness, and it is worth seeing it for what it is: output formatting, not a second grammar. The machine still says stem + ta; the mouth then simplifies the collision. Say nom-ta quickly a few hundred times over a few centuries and you get nonda too.

One genuine irregular: 行く iku → 行った itta (not iita).

The て form is the same change

Swap the final -a for -e and you have the て form, the most useful form in the language:

past	て form
書いた kaita	書いて kaite	writing / and wrote
飲んだ nonda	飲んで nonde
話した hanashita	話して hanashite

Learn the past and you have learned て free. て does an enormous amount of work:

linking clauses: 起きて、食べて、行きました — I got up, ate and went
progressive: 食べている — am eating
requests: 待ってください — please wait
permission: 食べてもいい — you may eat

た is not only the past

The -ta form marks completion, which is not the same thing as past time:

分かりました wakarimashita — "I understand" (said now; the understanding has just completed) バスが来た！ basu ga kita! — "the bus is here!" 疲れた tsukareta — "I'm tired" (the tiring has happened)

Translating these as English pasts produces the odd stiffness that marks a learner.

There is no perfect, and no past habitual

English	Japanese
I ate	食べました
I have eaten	食べました
I had eaten	食べました (context, or 食べていました)
I used to eat	よく食べていました — "was eating often"

For "have you ever…", Japanese uses ~たことがある: 日本に行ったことがあります — I have been to Japan (at some point in my life).

Polite past negative

The one clunky corner of politeness: 食べませんでした tabemasen deshita — "did not eat". Plain form is the tidy 食べなかった tabenakatta.

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