The た-form is the plain past tense - the plain-form equivalent of ました. If you already know the て-form, you already know how to build it.
Building it
Take the て-form and change て to た (or で to だ).
| て-form | た-form |
|---|---|
| たべて | たべた |
| のんで | のんだ |
| いって | いった |
| して | した |
| きて | きた |
That is the whole rule. If the て-form is not yet automatic, fix that first - it is the highest-value form in the language: the te-form.
Three patterns worth knowing
1. ～たことがあります - I have done (experience)
日本に 行ったことが あります。- I have been to Japan. すしを 食べたことが ありません。- I have never eaten sushi.
Note it is about experience, not about the past generally. For “I went to Japan last year” you use いきました, not the experience pattern.
This distinction is examined at N4 and appears in JFT-Basic distractors.
2. ～たり～たりします - doing things like A and B
休みの日は 本を 読んだり、そうじを したり します。
A non-exhaustive list: these among other things. Both verbs take た-form plus り, and the sentence ends with します in the appropriate tense.
3. ～たあとで - after doing
仕事が 終わったあとで、買い物を します。
Contrast with ～まえに (before), which takes the dictionary form rather than the た-form: 仕事が 終わる まえに. The two halves of this pair use different forms, which is exactly why exams like it.
Also common
～たら - one of the four conditionals: 雨が ふったら、行きません。See Japanese conditionals.
～たほうがいいです - you had better: 早く 帰ったほうが いいです。A staple of advice, and a polite way to suggest something at work.
Plain past in real speech
In casual conversation, past statements simply end in た: 食べた? (did you eat?), 見た! (I saw it!).
Keep this for friends. With colleagues and strangers, ます-form remains the safe choice - see the plain form for when the switch is appropriate.
How to drill it
Describe yesterday, out loud, every evening for a week:
六時に 起きた。ごはんを 食べた。仕事に 行った。同僚と 話した。
Then convert each to the polite past, then to the negative past. Five minutes a day, and the conversion becomes automatic rather than calculated - which is what exam conditions require.