Quite often “とき” is the first word that you learn to say “when” in Japanese. The kanji (時) we have seen in previous examples as the time marker.
とき is used with the standard form of the verb. Note that we need the target particle に to mark the specific time of the action.
担当者がいないときに、待合室で待ってください。
Please wait in the waiting room when the person in charge is not here.
大学を卒業して仕事を始めたときに、毎日がつらかった。
When I graduated from university and started work every day was tough.
Quite often the target particle following とき is omitted.
図書館で勉強していたとき、祐介はいたよ。
When I was studying in the library Yusuke was there.
Using Nouns and Adjectives
Because とき functions grammatically as a noun (meaning “time”), we connect other words to it using standard noun-connection rules. This means adding の for nouns, な for na-adjectives, and leaving i-adjectives as they are.
子供のとき、よくこの公園で遊びました。
When I was a child, I often played in this park.
暇なときに、本を読みます。
When I am free, I read books.
寂しいとき、家族に電話します。
When I am lonely, I call my family.
The Tense Trap: Before vs. After
One of the most important rules to remember is that the tense of the verb before とき changes the chronological order of events. If the verb before とき is in the present/dictionary form, the action has not finished yet (it means “Before X”). If it is in the past form, the action is completed (it means “After X”).
日本に行くとき、カバンを買った。
When I was going to Japan (before I arrived), I bought a bag.
This implies you bought the bag in your home country to prepare for the trip.
日本に行ったとき、カバンを買った。
When I went to Japan (after I arrived), I bought a bag.
This implies you bought the bag while you were actually in Japan.
Modifying Nouns and Other Uses
We can also use とき to modify the noun (something we can’t do with conditionals).
彼と会った時の第一印象がよかった。
I had a good first impression when I met him (the first impression of the time I met him was good).
年を取った親を介護する時がいつか来るだろう。
The time will come when you will look after your parents.