-아서/어서
because; and then
Two meanings in one ending, and three restrictions that make it the most error-prone connector at this level. No tense before it, no commands after it.
Build it
Pick a verb
When to use it
The first meaning is cause: 배가 고파서 밥을 먹었어요. The second is sequence with a link — the first action sets up the second, and the two share a subject: 친구를 만나서 영화를 봤어요 (met my friend and, together with them, saw a film). Compare -고, which reports two actions with no such connection.
Vowel selection copies -아요/어요 exactly, merges included: 가 becomes 가서, 보 becomes 봐서, 마시 becomes 마셔서, 하다 becomes 해서.
Now the restrictions. Tense never appears before it — 갔어서 does not exist; the past goes on the final verb. And no command or suggestion may follow it: for “it’s cold, so close the window,” Korean requires -(으)니까. What -아서 does own outright is the set of fixed social phrases: 만나서 반갑습니다, 늦어서 죄송합니다.
How to attach it
| Stem type | Form | Example |
|---|---|---|
| Last vowel ㅏ or ㅗ | -아서 | 좋다 → 좋아서 |
| Identical vowel merges | -아서 absorbed | 가다 → 가서 |
| Any other vowel | -어서 | 먹다 → 먹어서 |
| 하다 verbs | 해서 | 피곤하다 → 피곤해서 |
| 이다 | 이라서 / 여서 | 학생이라서 |
| Never | no tense before it | 갔어서 does not exist |
| Never | no command after it | 추우니까 닫으세요 — not 추워서 |
In use
배가 아파서 병원에 갔어요.
I had a stomachache, so I went to the hospital.
시간이 없어서 택시를 탔어요.
I didn’t have time, so I took a taxi.
친구를 만나서 같이 밥을 먹었어요.
I met my friend and we ate together.
날씨가 좋아서 산책했어요.
The weather was nice, so I went for a walk.
학생이라서 할인을 받았어요.
I got a discount because I’m a student.
너무 피곤해서 일찍 잤어요.
I was so tired that I went to bed early.
Compared with
-(으)니까
Both give reasons. -아서 presents the cause as a plain fact; -(으)니까 presents it as the speaker’s own reasoning, which is why only -(으)니까 can be followed by a command or a suggestion.
추워서 창문을 닫았어요. (statement) / 추우니까 창문을 닫으세요. (request)
-고
-고 lists actions with nothing tying them together; -아서 means the first leads into the second, and both clauses must share a subject.
친구를 만나고 영화를 봤어요. (two separate things) / 친구를 만나서 영화를 봤어요. (met, and watched with them)
-기 때문에
A heavier, more formal way of giving a cause, common in writing. Like -아서, it cannot be followed by a command.
비가 와서 못 갔어요. (speech) / 비가 왔기 때문에 가지 못했습니다. (writing)
Watch out
어제 아팠어서 학교에 못 갔어요.
어제 아파서 학교에 못 갔어요.
-아서 never carries tense. The past is expressed once, on the final verb, and the time word 어제 does the rest.
시간이 없어서 빨리 오세요.
시간이 없으니까 빨리 오세요.
A request or suggestion cannot follow -아서. Switch to -(으)니까 whenever the second clause tells someone to do something.
비가 오아서 못 갔어요.
비가 와서 못 갔어요.
오 + 아서 merges into 와서, exactly as 와요 does in the present. The same happens with 보다 → 봐서.
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