-아서/어서: Because, But Only for Statements

-아서/어서 presents a cause as an established fact. You are not arguing a point; you are reporting how the world went. A happened, therefore B.

That single property explains every rule below.

The test

Look at the second half of your sentence — the part after “because.”

Is it a plain statement? -아서/어서 works, and is usually the more natural choice.
Is it a command, request or suggestion? You need -(으)니까 instead.

비가 와서 못 갔어요.          ✅  It rained, so I couldn't go.
비가 와서 우산 가져가세요.     ❌  ungrammatical — command after -아서
비가 오니까 우산 가져가세요.   ✅  It's raining, so take an umbrella.

The second line is not “slightly unnatural.” A Korean speaker hears it as broken. Commands are addressed to someone, and -아서/어서 does not point at a listener — it just reports.

No past tense, ever

The tense lives in the final verb, never in the -아서 clause.

If you catch yourself writing 았어서 or 었어서, it is wrong every single time. This is one of the few Korean rules with no exceptions at all.

Apologies and thanks are frozen here

These take -아서/어서 and nothing else:

• 늦어서 죄송합니다 — Sorry I’m late.
• 도와주셔서 감사합니다 — Thank you for helping.
• 기다리게 해서 미안해요 — Sorry to keep you waiting.

Swap in -(으)니까 and the apology curdles. 늦으니까 죄송합니다 reads as “since I’m late, I suppose I’m sorry” — explaining rather than apologising. Native speakers notice immediately, and socially this is the costliest mistake with this pair.

The other job it does

Same form, different function: -아서/어서 also links two actions where the first sets up the second.

• 은행에 가서 돈을 찾았어요 — I went to the bank and withdrew money.
• 친구를 만나서 밥을 먹었어요 — I met my friend and we ate.

The going and the withdrawing are one connected event — you withdrew money at the bank you went to. Compare 은행에 가고 돈을 찾았어요, which sounds like two unrelated things that merely happened in order.

Context separates the two uses without effort. Nobody confuses them in practice.

Summary

Ask yourself one question before you speak: am I telling this person to do something? If no, -아서/어서 is almost certainly right.