-아서/어서

-아서/어서 links two clauses to show a reason ("so, because") or a closely connected sequence ("did A and then B with/at it").

이유·원인Reason & cause

Han: -아서/어서 attaches to the plain stem even when the reason is in the past; tense is marked only on the final verb.

Examples

비가 와서 집에 있었어요.It rained, so I stayed at home.

시간이 없어서 아침을 못 먹었어요.I didn't have time, so I couldn't eat breakfast.

사과를 씻어서 먹었어요.I washed the apple and ate it.

도서관에 가서 공부했어요.I went to the library and studied there.

V/A+아서/어서 (ㅏ,ㅗ → 아서; other vowels → 어서; 하다 → 해서)

가다 → 가서, 먹다 → 먹어서, 피곤하다 → 피곤해서

Do not put past -았/었- or -겠- before -아서/어서; tense is shown on the final verb: 늦어서 (not 늦었어서).
In the "because" meaning, the sentence cannot end with a command or suggestion (-(으)세요, -(으)ㅂ시다, -자); use -(으)니까 instead.
In the "and then" meaning, both actions have the same subject and the second happens at the place or with the object of the first: 친구를 만나서 영화를 봤어요 (watched a film with that friend).
It is the natural choice for reasons in thanks and apologies: 늦어서 죄송합니다, 도와주셔서 감사합니다.

Wrong: ~~어제 늦게 잤어서 피곤해요.~~

Right: 어제 늦게 자서 피곤해요.

-아서/어서 attaches to the plain stem even when the reason is in the past; tense is marked only on the final verb.

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