English "when" is tense-neutral: "when I go to Japan" and "when I went to Japan" differ only in the main verb's tense — the "when" itself is inert. Japanese is the opposite. In a とき clause, the tense you put on the verb before とき is doing critical work: it tells you whether that event happens before or after the main event. 行くとき and 行ったとき are not present versus past — they are "on the way there" versus "once there." This is one of the highest-value distinctions in Japanese grammar, and English simply leaves it to context. This page makes the rule vivid and shows exactly why it works.
とき is a noun — so its clause is a modifier
The key to everything is that 時（とき） is literally the noun "time." So a とき clause is a modifying (relative) clause sitting on the noun 時 — "the time when X." And modifying clauses in Japanese take relative tense: their verb is timed against the head noun's own moment, not against "now." That single fact is the whole rule. Once you see とき as "the time-when," it follows automatically that the とき-verb marks its event relative to the main clause.
The load-bearing contrast: 行くとき vs 行ったとき
Take one pair and let it teach the whole rule. Both sentences below end in the same past main verb — 買（か）った ("bought"). Only the tense inside the とき clause changes. Watch the meaning flip completely:
アメリカへ行くとき、スーツケースを買った。
amerika e iku toki, sūtsukēsu o katta
When I was going to America (before I left), I bought a suitcase — here, before departure.
アメリカへ行ったとき、スーツケースを買った。
amerika e itta toki, sūtsukēsu o katta
When I got to America (once I was there), I bought a suitcase — over there, after arriving.
The suitcase in the first sentence was bought at home, before the trip; in the second, in America, after arriving. Nothing in the main clause changed — 買った is past in both. The entire difference is carried by 行く (the trip not yet complete at the moment of buying → before) versus 行った (the trip complete before the buying → after). Line up the logic:
| とき-clause | Relative timing | Meaning |
|---|---|---|
| 行くとき (non-past) | the going is not yet done at the main event | on the way / about to go — before arriving |
| 行ったとき (past た) | the going is already done at the main event | once there — after arriving |
Here are the two required base readings on their own, so the pattern sticks:
帰るとき、牛乳を買う。
kaeru toki, gyūnyū o kau
On my way home (as I head back), I'll buy milk. (bought before arriving home)
帰ったとき、誰もいなかった。
kaetta toki, dare mo inakatta
When I got home (after arriving), there was no one there.
帰るとき puts the buying on the journey home — you're not home yet. 帰ったとき puts the "no one there" after you've arrived — the arriving is complete. You could not swap them: you can't buy milk after getting home under 帰るとき, and you can't find an empty house before you've arrived under 帰ったとき.
Relative to the main event, not to now
The most important and least obvious consequence: the とき-clause's tense answers to the main clause, not to the present moment. So a past た can sit inside a future sentence — as long as the とき-event finishes before the main event does. This looks wrong to English eyes and is completely natural in Japanese:
東京に着いたとき、電話するね。
Tōkyō ni tsuita toki, denwa suru ne
When I get to Tokyo, I'll call you.
着いた is past, yet the whole sentence is about the future (電話する, "I'll call"). Why past? Because the arriving finishes before the calling — relative to the main event, arrival is already complete. English forces "when I get there" (present), so the た here is a genuine mind-bender until the relative-tense rule clicks. Anchor to the main event and ask "before or after?", and 着いたとき is obviously right: you call after you've arrived.
Contrast with the "as I'm doing it" reading, where the action is still unfolding at the main moment and takes non-past:
家を出るとき、鍵をかけた。
ie o deru toki, kagi o kaketa
As I left the house, I locked the door. (the locking happens at the moment of leaving)
ドアを開けたとき、猫が飛び出してきた。
doa o aketa toki, neko ga tobidashite kita
When I opened the door, the cat came bolting out. (after the door was open)
出るとき locks the door in the act of leaving — leaving isn't finished, so non-past. 開けたとき releases the cat after the door is open — opening is done, so past た. Same pattern, every time.
States and nouns: 子供のとき, 暇なとき, 若いとき
When the とき-clause is a state rather than an action, there's no before/after puzzle — states simply overlap the main event. These link the way nouns and adjectives normally do: noun + の, な-adjective + な, い-adjective plain.
子供のとき、この町に住んでいた。
kodomo no toki, kono machi ni sunde ita
When I was a child, I lived in this town.
暇なとき、電話してください。
hima na toki, denwa shite kudasai
Call me when you're free.
若いとき、もっと勉強すればよかった。
wakai toki, motto benkyō sureba yokatta
I wish I'd studied more when I was young.
And a non-past state can pin a habitual "whenever" reading:
日本語が分からないとき、辞書を使う。
nihongo ga wakaranai toki, jisho o tsukau
When I don't understand Japanese, I use a dictionary.
Common mistakes
Mistake 1 — Copying the main clause's tense (the core error). English speakers match the とき-clause to the main verb. If the とき-event was complete before the main event, it must be past — even against a past main clause of the "same time" feel.
❌ 日本に行くとき、写真を撮った。
Wrong for 'took photos while there' — 行くとき means 'before arriving,' so you weren't in Japan yet. Use 行ったとき.
✅ 日本に行ったとき、写真を撮った。
nihon ni itta toki, shashin o totta
When I went to Japan (while there), I took photos.
Mistake 2 — Using past た for a "before / as you do it" action. If the main action happens at the moment of, not after, the とき-clause stays non-past.
❌ 出かけたとき、電気を消して。
Wrong — 出かけたとき is 'after you've already left,' when you're no longer there to switch off the lights. Use 出かけるとき.
✅ 出かけるとき、電気を消して。
dekakeru toki, denki o keshite
Turn off the lights when you leave (as you go out).
Mistake 3 — Non-past とき for an event that must finish first. "When I arrive, I'll call" needs た, because arriving completes before calling — even though the sentence is about the future.
❌ 東京に着くとき、電話する。
Odd — 着くとき is 'as I'm arriving / approaching.' For 'once I've arrived, then call,' the arrival must be complete: 着いたとき.
✅ 東京に着いたとき、電話する。
Tōkyō ni tsuita toki, denwa suru
When I get to Tokyo, I'll call.
Mistake 4 — Dropping の / な before とき. A noun links with の and a な-adjective with な; you can't butt them bare against とき.
❌ 子供とき、ここに住んでいた。
Wrong — a noun needs の: 子供のとき (and な-adjectives need な: 暇なとき).
✅ 子供のとき、ここに住んでいた。
kodomo no toki, koko ni sunde ita
When I was a child, I lived here.
Key takeaways
- とき is the noun 時 ("time"), so its clause is a modifier and takes relative tense — timed against the main event, not against now.
- 行くとき (non-past) = the event is not yet complete at the main moment → before / on the way. 行ったとき (past た) = already complete → after / once there.
- The reference point is the main clause, so a past た can live inside a future sentence (東京に着いたとき、電話する).
- Don't copy the English main-verb tense — re-anchor to the main event and ask "before, during, or after?"
- States overlap the main event and link normally: 子供のとき, 暇なとき, 若いとき.
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