A2
とき — when
How the Japanese word とき builds a “when” clause, why the verb before it must be plain, and how its tense changes the meaning.
とき is the noun time. A clause in front of it says which time, exactly as an adjective would.
子供のとき、京都に住んでいました。
Kodomo no toki, Kyōto ni sunde imashita.
When I was a child, I lived in Kyoto.
暇なとき、映画を見ます。
Hima na toki, eiga o mimasu.
When I’m free, I watch films.
Because it is a noun, a preceding noun needs の and a な-adjective needs な — the same joining rules as anywhere else.
The verb before it is plain
Never ます, however polite the sentence.
日本に行きましたとき、写真を撮りました。
日本に行ったとき、写真を撮りました。
The clause modifying とき takes the plain form. Politeness lives in the final verb only — see the plain form.
Its tense changes the meaning
This is the part worth slowing down for. The form before とき says whether the action was complete at that time, and the difference is real.
日本に行くとき、カメラを買いました。
Nihon ni iku toki, kamera o kaimashita.
I bought a camera when I went to Japan.
日本に行ったとき、カメラを買いました。
Nihon ni itta toki, kamera o kaimashita.
I bought a camera when I was in Japan.
行くとき — the going had not happened, so the camera was bought before departure. 行ったとき — the going was complete, so it was bought there. One character, two different holidays.
Against たら
とき states when something happened or happens. たら sets a condition. They overlap in English when, but とき never carries if — see たら.