Here's how I see the different aspects of -(아/어)서, -(으)니까, and 때문에. - -(아/어)서 = indicates a direct and immediate cause, which is often situational rather than logical. => Because; because of; as. - -(으)니까 = gives a reason, or a personal situation that gives rise to something. It is more detached from the effect clause. => since; given (a reason clause); (a clause), so ... - 때문에 = gives a direct, logical reason. => because; on account of. So -서 and 때문에 gives direct reason that is tightly connected with the main clause. The difference is that 때문에 is more logical. Because of the tight connection, they don't work well in an imperative or exclamatory sentence. -니까 makes the clause more detached from the main clause. It can be very circumstantial rather than a generally accepted cause-and-effect. It is well suited to imperative sentences. (imperative sentence example) - 보기 싫어서 나가! (X) - 보기 싫기 때문에 나가! (X) - 보기 싫으니까 나가! (O) (exclamatory sentence example) - 저렇게 돈이 많기 때문에 얼마나 좋을까 (X) - 저렇게 돈이 많아서 얼마나 좋을까 (okay, but not the best) - 저렇게 돈이 많으니(까) 얼마나 좋을까 (O)

"I'm learning Korean because I work with Korean people." 저는 한국인들과 일해서 한국어를 공부합니다. "I can't go out this weekend because I'll be too busy" 저는 너무 바빠서 이번 주말에 놀러가지 못합니다. "The movie was boring, so I turned off the TV" 그 영화는 너무 지루해서 저는 티비를 껐습니다. "I can't buy this shirt because I don't have enough money for it" 저는 돈이 충분하지 않아서 이 셔츠를 살 수 없습니다. "I'm not good at A, so I don't like doing A" ( I couldn't think of an example ) 저는 a를 잘 하지 못해서 a를 하기 싫어합니다. "I'm tired, so let's just get out of here" 저는 피곤해요, 그니까 여기서 나가요(basically two different sentences; awkward when you use any of 서 or 하니까) "You look better with short hair, so you should get a haircut" 너는 짧은 머리가 더 잘 어울리니까 머리를 잘라야한다. "It's cold outside, so let's go out when it's warmer." 밖은 추우니까 따뜻해지면 나가자. "Since you're not listening, I'll stop talking" 니가 안듣고 있으니까 나는 그만 말할래. Actually I don't think there is specific difference between 서 and 니까. But in some sentences that would sound a bit awkward wen you use one of them or even both of them. So as you said, I think it is the best to practice with many examples.