-(으)ㄴ/는 김에

Use it when you take the chance of doing one thing to also do another thing: 'while I'm at it, I'll also…'.

시간·순서Time & sequence

Han: 김에 implies you deliberately used the chance to do something extra; a chance meeting is not chosen, so use 길에 ('on the way').

Examples

말이 나온 김에 이번 주말에 같이 영화 보러 가요.Since we're on the subject, let's go see a movie together this weekend.

방을 청소하는 김에 창문도 닦았어요.While I was cleaning the room, I wiped the windows as well.

부산에 출장 간 김에 오랜만에 친구도 만났어요.While I was in Busan on a business trip, I also met a friend I hadn't seen in a long time.

우체국에 가는 김에 이 편지도 좀 부쳐 줄래?Since you're going to the post office anyway, could you send this letter too?

V+는 김에 (action in progress / about to happen) · V+(으)ㄴ 김에 (action already done)

가다 → 가는 김에 / 간 김에; 나오다 → 나온 김에

Attach it to action verbs; with adjectives or 이다 it usually sounds unnatural.
Choose 는 김에 when the first action is happening or about to happen, and (으)ㄴ 김에 when it is already done; the tense of the whole sentence goes at the end (no 갔는 김에).
The second clause is a deliberate extra action, so it does not fit accidental events; it can freely end in a request or suggestion.
Both clauses usually share the same subject.

Wrong: ~~편의점에 가는 김에 우연히 선생님을 만났어요.~~

Right: 편의점에 가는 길에 우연히 선생님을 만났어요.

김에 implies you deliberately used the chance to do something extra; a chance meeting is not chosen, so use 길에 ('on the way').

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