-아서/어서 — reason, cause, and 'and so'
Korean has several ways to say 'because', but the workhorse for everyday spoken reasons is -아서/어서, attached to the verb stem of the cause clause. 비가 와서 못 가요. = 'It's raining, so I can't go.' / 'Because it's raining, I can't go.' The same ending can also mean 'and (then)' for sequential actions where the second one builds on the first: 앉아서 책을 읽어요. = 'I sit down and read a book.' Two important rules: (1) the subject of both clauses must be the same when used for sequence, and (2) you cannot put past-tense -았/었- before -아서/어서 — the time of the cause is read off the final verb.
How to attach: 아 vs. 어 (vs. 여)
Look at the last vowel of the verb stem. If it's ㅏ or ㅗ, add -아서: 가다 (go) → 가서, 오다 (come) → 와서, 좋다 (be good) → 좋아서. Anything else takes -어서: 먹다 (eat) → 먹어서, 읽다 (read) → 읽어서, 있다 (exist) → 있어서. The verb 하다 is irregular and becomes 해서: 공부하다 → 공부해서. Vowel-stem contractions are normal: 가다 + 아서 → 가서 (not 가아서), 서다 + 어서 → 서서, 오다 + 아서 → 와서.
Causal use — 'because / so'
Put the cause clause first with -아서/어서, then the result. 늦어서 미안해요. = 'I'm sorry I'm late' (lit. 'because I'm late, I'm sorry'). 배가 고파서 밥을 먹어요. = 'I'm hungry, so I'm eating.' 시간이 없어서 못 갔어요. = 'I didn't have time, so I couldn't go.' Notice the past tense lives only on the FINAL verb — 없어서 stays present-shaped even though the whole event is past. One important restriction: -아서/어서 cannot be used with commands or suggestions like 'Let's…'. For those you'll use -(으)니까 (a different 'because' you'll meet later).
Sequential use — 'and (then)'
When the two actions share a subject AND the second flows out of the first, -아서/어서 also means 'and so / and then'. 앉아서 기다려요. = 'I sit down and wait.' 친구를 만나서 영화를 봤어요. = 'I met a friend and (we) watched a movie.' 학교에 가서 공부해요. = 'I go to school and study.' This is different from -고, which just lists two actions in order without a built-in connection: 밥을 먹고 잤어요. = 'I ate and (then) slept' (two unrelated events). Use -아서 when the second action depends on or builds from the first; use -고 for plain sequence.
Same-subject restriction (for the sequence reading)
For the 'and then' meaning, both clauses must share a subject. 저는 집에 가서 쉬어요. = 'I go home and rest' — same I. You can't say 친구가 와서 저는 갔어요 to mean 'My friend came and I left' as a sequence. (As a CAUSE — 'My friend came, so I left' — that sentence is fine; the restriction only applies to the sequential reading.) When the subjects differ and you want plain sequence, use -고: 친구가 오고 저는 갔어요.
No -았/었- before -아서/어서
This is the trap learners fall into. Even when the cause is clearly past, the -아서/어서 clause stays present-shaped. Right: 비가 와서 못 갔어요. ('It rained, so I couldn't go.') Wrong: 비가 왔어서 못 갔어요. The final verb 갔어요 carries the past tense for the whole sentence; the cause clause borrows that timing. The same goes for the future: 시간이 없어서 못 갈 거예요. ('I won't have time, so I won't be able to go.') — the -(으)ㄹ 거예요 at the end does the work.
