Joining sentences: 고, 서, 지만, 니까
Korean glues clauses instead of starting new sentences
고 — and, in sequence or in a list.
밥을 먹고 커피를 마셨어요. Two actions, loosely ordered.
아/어서 — and *therefore*, or and *then* with a real connection.
비가 와서 안 갔어요. (It rained, so I didn't go.) The clauses must be linked; you cannot use 서 for unrelated events.
지만 — but.
비싸지만 맛있어요. Straightforward, and safe with any speech level.
니까 — because, with a nuance.
비싸니까 안 사요. 니까 gives a reason the listener is expected to accept, and it is the one you use before commands: 추우니까 문 닫으세요. 서 cannot do that.