A2
아/어서 and (으)니까 — because
The difference between 아서 and 니까 in Korean, which one works before a command or suggestion, and why 아서 takes no tense.
Both mean because, and both put the reason first.
비가 와서 안 갔어요.
Biga waseo an gasseoyo.
I didn’t go because it was raining.
시간이 없으니까 빨리 가요.
Sigani eopseunikka ppalli gayo.
We haven’t got time, so let’s go quickly.
The rule that decides between them
아/어서 cannot be followed by a command, a request or a suggestion. 니까 can. This is not a matter of nuance — the wrong one is ungrammatical.
비가 와서 우산을 가져가세요.
비가 오니까 우산을 가져가세요.
The second clause is a request, so the reason must take 니까. 아서 only ever introduces a statement.
Stick to 니까 whenever the sentence ends in 세요, 을까요, 읍시다 or a plain imperative.
아서 takes no tense
The past goes on the final verb only, however clearly the reason happened first.
아팠어서 못 갔어요.
아파서 못 갔어요.
아서 never carries 았/었. 니까 can — 아팠으니까 is correct — which is a second, quieter difference between them.
Tone
니까 presents the reason as something the listener should accept, which makes it slightly assertive. Apologising with it can sound like arguing, so an apology usually takes 아서.
늦어서 죄송합니다.
Neujeoseo joesonghamnida.
I’m sorry I’m late.
늦으니까 죄송합니다 would sound like explaining why you are entitled to be late.
Between sentences
그래서 begins a new sentence; 왜냐하면 introduces a reason after the fact, as English because can when it trails.
비가 왔어요. 그래서 안 갔어요.
Biga wasseoyo. Geuraeseo an gasseoyo.
It rained. So I didn’t go.