とき

「とき」indicates the time or situation in which the event in the following clause occurs.
Note:
- The subject of the first and second clauses can be different. The subject of the first clause is marked with が. If both clauses share the same subject, this subject can be placed at the beginning of the sentence. Examples:
　・私が晩ご飯を食べているとき、友達がうちへ来ました。(When I was having dinner, my friend came over.)
　・若いとき、お母さんは看護士でした。(When she was young, my mother was a nurse.)
　・お母さんは若いとき看護士でした。(My mother was a nurse when she was young.)
- The temporal relationship between the first and second clauses is expressed by the tense of the verb in the first clause, regardless of the overall tense of the sentence. Specifically:
　+ When the verb in the first clause is in the past tense ➔ the first clause happened before the second clause. Examples:
　・日本へ来たとき、空港で写真を撮りました。(When I arrived in Japan, I took photos at the airport.)
　+ When the verb in the first clause is in the present tense ➔ the first clause happens after the second clause. Examples:
　・日本へ来るとき、空港で写真を撮りました。(Before coming to Japan, I took photos at the airport.)
- When the second clause is in the past tense, and the first clause is a verb indicating a state (いる、ある...), it can be in either present or past tense. Examples:
　・部屋にいるとき、電話しました。(I called when I was in the room.)
　・部屋にいたとき、電話しました。(I called when I was in the room.)
- When the second clause is in the past tense, and the first clause is an adjective or noun, the adjective or noun can be in either present or past tense. If you want to emphasize the past state of the first clause, you can use it in past tense. Examples:
　・若いとき、よく泳ぎました。(I used to swim a lot when I was young.)
　・若かったとき、よく泳ぎました。(I used to swim a lot when I was young.)
- You can add the particle に after とき to form ときに.
- The kanji for 「とき」is 時, however, in official documents like legal texts, there is a distinction in usage between the two characters. When とき indicates a point in time, 時 is used, while when it indicates a situation, hiragana is used. Examples:
　・図書館で本を借りるとき、カードがいります。(Situation)
　・夕飯の時にこのレストランに来てほしい。(Time)