"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.