〜とき (tense inside the とき clause)

"When…" — the tense of the verb before とき shows whether the とき action is complete relative to the main action.

とき means "when", but the form of the verb before it tells you the order of events. Dictionary form + とき: the main action happens BEFORE or DURING the とき action (日本へ行くとき、お土産を買った = I bought souvenirs on the way, before arriving). た-form + とき: the main action happens AFTER the とき action is finished (日本へ行ったとき、京都を見た = after I got to Japan, I saw Kyoto). The tense at the end of the sentence shows when the whole thing happened; the form before とき only shows which came first.

Any time you say "when": daily routines (寝るとき歯を磨く), past stories (子供のとき), instructions (分からないときは聞いてください). The dictionary/た distinction matters most with movement and change verbs: 行く, 来る, 帰る, 着く, 出る, 入る.

• V-dictionary + とき: main action before or during (家を出るとき、鍵をかける)
• V-た + とき: main action after the とき action is completed (家に帰ったとき、手を洗う)
• V-ている + とき: during an ongoing action (テレビを見ているとき)
• い-adjective + とき, な-adjective + なとき, Noun + のとき: 忙しいとき, 暇なとき, 子供のとき
• The final verb sets the overall tense; the とき clause is relative to it

1. 01

   日本へ行くとき、空港でお土産を買った。

   にほんへ いくとき、くうこうで おみやげを かった。

   When I was going to Japan, I bought souvenirs at the airport (before arriving).

   Dictionary form: the buying happened before arrival (at the departure airport).

2. 02

   日本へ行ったとき、京都を見に行った。

   にほんへ いったとき、きょうとを みに いった。

   When I went to Japan, I went to see Kyoto (after arriving).

   た-form: the sightseeing happened after arriving in Japan.

3. 03

   子供のとき、よく川で泳いだ。

   こどもの とき、よく かわで およいだ。

   When I was a child, I often swam in the river.

   Noun + のとき: a period of life.

4. 04

   家を出るとき、電気を消すのを忘れないでください。

   いえを でるとき、でんきを けすのを わすれないでください。

   When you leave the house, don't forget to turn off the lights.

   You turn off the lights before actually going out: dictionary form + とき.

5. 05

   駅に着いたとき、電車はもう出ていた。

   えきに ついたとき、でんしゃは もう でていた。

   When I got to the station, the train had already left.

   た-form: the arriving was complete when the speaker saw the train gone.

• 〜たら

  たら also means "when" for the future, but always with the order "A finished, then B". とき can express both orders. Compare side by side

• 〜間・〜間に

  間に means "during the time that" and needs a continuing state or action. とき is a point or period "when". Compare side by side

• 〜場合は

  場合 is "in the case that", used for possibilities and rules. とき is for real times and events. Compare side by side

• ✗

  家に帰るとき、手を洗います。(meaning: after getting home)

  ✓

  家に帰ったとき、手を洗います。

  Why

  Washing hands happens after you get home, so the completed た-form is needed. 帰るとき would mean "on the way home".

• ✗

  日本へ行ったとき、空港でお土産を買いました。(bought at the home airport before leaving)

  ✓

  日本へ行くとき、空港でお土産を買いました。

  Why

  The buying was before arrival, so the dictionary form is required, even though the whole sentence is past.

• ✗

  暇のとき、映画を見ます。

  ✓

  暇なとき、映画を見ます。

  Why

  な-adjectives take な before とき; の is for nouns (子供のとき).

Verbs of state (いる, ある, 分かる, 忙しい) usually take the dictionary form regardless of time: 日本にいるとき, 分からないとき. With verbs that take time (食べる, 読む), dictionary form + とき means "while": ご飯を食べるとき、テレビを見ない. 〜たとき can be shortened to 〜たら in casual speech for past sequences, but とき is the form tested for the order of events.