-아서/어서 vs -(으)니까

Beginner · TOPIK I

Both give a reason, but -아서/어서 states a cause in a neutral, natural way, while -(으)니까 presents the speaker's own reasoning and can be followed by commands and suggestions.

-아서/어서A connective ending used to indicate that the preceding event and the following one happened sequentially.-(으)니까A connective ending used to emphasize that the preceding statement is the cause, reason, or premise for the following statement.

Rules of thumb

If the main clause is a command or suggestion (-(으)세요, -(으)ㅂ시다, -(으)ㄹ까요), only -(으)니까 works.
-았/었 cannot come before -아서 (* 늦었어서), but it can before -(으)니까 (늦었으니까).
Apologies, thanks and greetings use -아서, not -(으)니까 (늦어서 죄송합니다, 만나서 반가워요).
-(으)니까 often appears when the reason is obvious to both sides or when the speaker is trying to persuade.
-(으)니까 also means 'when I did…, I found that…' (집에 가 보니까 아무도 없었어요).

Example sentences

-아서/어서
It rained, so I stayed home.
-(으)니까
It's raining, so take an umbrella.
비가 와서 우산을 가져가세요.
Incorrect
It's raining, so take an umbrella. [Wrong: -aseo/eoseo can't be followed by a command; use -(eu)nikka]
-아서/어서
Sorry I'm late.
-(으)니까
I slept well last night, so I'll be fine today.

Often confused with

← All confused pairs