-(으)ㄹ 때

Attaches to a verb or adjective to say at what time or in what situation something happens: 'when …'.

시간·순서Time & sequence

Han: The arrival was already finished when the friend was seen waiting, so -았/었을 때 is needed; 도착할 때 would mean 'as I was about to arrive'.

Examples

밥을 먹을 때 휴대폰을 보지 마세요.Don't look at your phone while you're eating.

시간이 있을 때 보통 영화를 봐요.When I have time, I usually watch movies.

한국에 처음 왔을 때 한국어를 하나도 몰랐어요.When I first came to Korea, I didn't know any Korean at all.

피곤할 때는 일찍 자는 게 좋아요.When you're tired, it's better to go to bed early.

V/A+(으)ㄹ 때

completed action: V/A+았/었을 때

N+때

먹다 → 먹을 때, 가다 → 갈 때, 춥다 → 추울 때, 오다 → 왔을 때, 방학 → 방학 때

If the action was already completed at that moment, use -았/었을 때: 집에 갈 때 = on the way home, 집에 갔을 때 = after getting home.
For future situations, do not put -(으)ㄹ 거 or -겠- before 때; the final verb shows the future: 내일 출발할 때 전화하세요.
With time nouns, 때 usually follows the noun directly (방학 때, 점심 때, 시험 때); for a role or state, use N일 때 (학생일 때).

Wrong: ~~공항에 도착할 때 친구가 기다리고 있었어요.~~

Right: 공항에 도착했을 때 친구가 기다리고 있었어요.

The arrival was already finished when the friend was seen waiting, so -았/었을 때 is needed; 도착할 때 would mean 'as I was about to arrive'.

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