~(으)니까, ~기 때문에, ~서... because....
~(으)니까 v(c) this is more often use when giving a reason with usually a ending which carries suggestion.. eg: 오늘은 바쁘니까 내일 만날까요? Because I'm busy today, shall we meet tomorrow? ~기 때문에 this is usually use for more serious reason, something more serious, formal or solid. Often for news etc.. eg: 그 식당은 너무 비싸기 때문에 손님많이있어요. ~아서/ 아사/ 애서 this is also use to show that the second half of the sentence is the reason why the first happens. eg: 지금 비가 와서 나가지 맙시다.