With (으)니까, the V/A before is the reason or the basis of judgement of the V that follows:
오늘은 피곤하니까 일찍 잘 거예요.
Today I’ll go to bed early because I’m tired.
제가 숙제 많이 있으니까 PC방에 못 가요.
Since I have a lot of homework, I’m not going to the PC room.
(When constructing sentences it may help to think of (으)니까 as ‘since’.)
-(으)니까 is used instead of 서 when the reason is personal or not commonly known. Compare:
비가 많이 왔어서 우산을 가지고 갔어요.
It was raining heavily, so I took an umbrella.
비가 많이 오니까 내일 만납시다.
Since it’s raining heavily, let’s meet tomorrow.
(으)니까 is used instead of 서 for suggestions:
지금은 밥을 먹어서 나중에 축구를 합시다.
지금은 밥을 먹으니까 나중에 축구를 합시다.
Since I’m eating a meal now, let’s play football later.
배가 많이 고프니까 먼저 밥 먹을게요.
I’m really hungry so I’ll eat first.
저번에 갔으니까 오늘은 안 가려고요.
Since I went the other day I’m not going to go today.
저는 조금 바쁘니까 나중에 전화해 주세요.
Since I’m a little busy please call me later.
버스가 늦으니까 집까지 걸었어요.
Because the bus was late I walked home.
이 기차는 복잡하니까 다음 기차를 타고 갈게요.
Since this train is crowded I’ll take the next train.
저는 공부해야 되니까 수영 안 해요.
I must study so I’m not going swimming.
날씨가 아직 시원하니까 산책을 합시다.
Since the weather is still cool, let’s go for a walk.