-으면 vs -(으)니까: If It Rains, or Because It Is Raining

Rain in the doorway

It is raining on the step now.

비가 오니까 들어가세요.
biga onikka deureogaseyo — It’s raining, so go in.

니까 hands them a reason. It can sit in front of a command. ㄹ drops: 살다 → 사니까.

Swap the hinge.

비가 오면 들어가세요.
biga omyeon deureogaseyo — If it rains, go in.

으면 is if / when. The rain is not claimed as now. If water is already on the step, 오면 들어가세요 is the failed line. 비가 와서 들어가세요 is a second failure: -아서 cannot precede a command.

A plan, not a puddle

비가 오면 우산 가져갈게요 — if it rains, I will take one. The rain is still a condition.

비가 오니까 우산 가져가요 — because it is raining, I am taking one. The rain is the reason.

Learners paste English because onto 면, and if onto 니까. The apology trap is worse. 늦으면 미안해요 is an if. 늦으니까 미안해요 sounds like a defence. The apology is 늦어서 미안해요. That rule does not move.

One test

Is the first half a condition that may still be open? 으면. Is it a reason you are handing them now, maybe with 세요 after it? 니까. If you only remember one failed line, remember 와서 들어가세요 and 오면 들어가세요 said in a live downpour. The live reason-plus-command is 오니까. 살면 가까워요 is an if about living there. 사니까 걸어가요 is because I live there — and the ㄹ is already gone. 시간 있으면 오세요 is still an if. 시간 없으니까 내일 해요 is a reason. Same clock, two hinges. A live puddle plus 세요 is never 오면 and never 와서. Do not leave thinking both are fine. English if/because will try to sell you that lie.