intermediate
Links two clauses as cause-and-effect, or as a connected sequence of actions.
Usage
Stem ends in ㅏ/ㅗ → 아서 (가다 → 가서).
All other vowels → 어서 (먹다 → 먹어서).
하다 verbs → 해서.
Never carries tense: say 늦었어서 ✗ → 늦어서 ✓. The tense lives in the final verb.
Example:
비가 와서 못 갔어요. (I couldn't go because it rained).
Nuance
Two distinct jobs, told apart by whether the actions are related in cause or in sequence.
Reason: 피곤해서 일찍 잤어요. (I slept early because I was tired).
Sequence: 시장에 가서 과일을 샀어요. (I went to the market and there bought fruit). The second action happens at or with the result of the first — this is why 가서, 와서, 앉아서, 만나서 are so common.
아/어서 cannot be followed by a command or a suggestion. 배고파서 밥 먹자 ✗ → use (으)니까 instead.
In Context
늦어서 죄송합니다.
I'm sorry for being late.
친구를 만나서 영화를 봤어요.
I met my friend and (then) we watched a movie.
값이 비싸서 안 샀어요.
I didn't buy it because it was expensive.