-(으)니까

-(으)니까 gives a reason, especially when you then tell, ask or suggest something to the listener.

이유·원인Reason & cause

Han: -아서/어서 cannot be followed by a command like -(으)세요; use -(으)니까 for a reason before a command.

Examples

비가 오니까 우산을 가져가세요.It's raining, so take an umbrella.

오늘은 바쁘니까 내일 만날까요?I'm busy today, so shall we meet tomorrow?

날씨가 추우니까 따뜻하게 입으세요.It's cold, so dress warmly.

시험이 끝났으니까 같이 놀러 가요.The exam is over, so let's go out and have fun together.

V/A+(으)니까

N+(이)니까

먹다 → 먹으니까, 가다 → 가니까, 학생 → 학생이니까

The second clause can be a command, request or suggestion (-(으)세요, -(으)ㅂ시다, -(으)ㄹ까요?); -아서/어서 cannot do this.
The past tense can come before it: 왔으니까, 끝났으니까.
Watch irregular stems: ㄹ drops (살다 → 사니까, 만들다 → 만드니까), and ㅂ becomes 우 (춥다 → 추우니까).
With thanks and apologies, use -아서/어서 instead: 늦어서 죄송합니다 (not 늦으니까 죄송합니다).

Wrong: ~~비가 와서 우산을 가져가세요.~~

Right: 비가 오니까 우산을 가져가세요.

-아서/어서 cannot be followed by a command like -(으)세요; use -(으)니까 for a reason before a command.

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