Reasons: ~(으)니까, ~아/어서
Express reasons and causes with two different patterns and their nuances.
Korean has two main ways to express 'because' / 'so', and the choice carries strong nuance. -(으)니까 emphasises the speaker's reasoning or judgement — used when you're explaining why you decided something or justifying an action: 비가 오니까 우산을 가져가세요 (since it's raining, take an umbrella). -아/어서 expresses a more objective cause-effect chain — used when the reason is just a fact: 비가 와서 우산을 가져왔어요 (because it rained, I brought an umbrella).
The critical rule: -아/어서 cannot be used with command or suggestion endings, while -(으)니까 can. 'Because it's raining, let's go home' must use -(으)니까: 비가 오니까 집에 가요. The opposite is sometimes flagged on TOPIK exams. Most learners default to -아/어서, which works for explanations but sounds wrong for instructions. Build the habit of using -(으)니까 when giving advice or suggestions — it's the more natural choice in those contexts.
~아/어서 — Because (natural cause/reason)
Verb/adjective stem + 아/어서 gives a reason. Cannot be used with imperative (주세요) or propositive (갈까요?) endings. Used for objective/natural reasons: 배고파서 밥 먹었어요 (I ate because I was hungry).
~(으)니까 — Because / Since (subjective reason + commands OK)
Verb/adjective stem + (으)니까 also gives a reason, but CAN be used before commands and suggestions. Often more subjective/emphatic: 추우니까 문 닫아 주세요 (It's cold, so please close the door).