아/어서 is fairly easy to understand, but 니까 is really tricky to understand because it doesn’t map to a 1 to 1 English translation. 니까 serves two primary purposes. Lets have a look
Firstly, both patterns express cause and effect.
- 채소를 별로 안 좋아해서 잘 안 먹어
- 채소를 별로 안 좋아하니까 잘 안 머거
Both say I don’t really like vegetables so I don’t usually eat them. The difference is that in general 아/어서 is fairly neutral. You don’t like something so its common sense you won’t eat it. But when you say 니까 it is more subjective and contains your personal opinion, or an attitude, and you’re using that as a basis of justification for whatever is in the second clause. As a result it is a lot more colloquial. It is also more appropriate to use (although not necessary) when the preceding clause is something both you and the listener is aware of. For example
- 너는 학생이니까 공부를 열심히 해야 한다. You are providing a subjective opinion, and you’re using that as a basis as justification for what you’re saying in the second clause.
- 차가 막히니까 늦었어. This is not natural because it is a very common sense and straight forward relationship. However you could use it if both you and the listener was aware of the traffic situation and you wanted to emphasis that the traffic congestion is the reason you’re late. Here you are kind of giving off a feeling of “why else would I be late?”
- 제가 잘못했으니까 제가 사과하겠습니다. Here it would be perfectly fine to use 해서 because its natural for people to apologize when they are the one who made the mistake. But because 니까 is used you are providing a subjective and person justification that you think you should apologize because it is your personal opinion that when they did something wrong they should apologize.
- 피곤했으니까 일찍 젭에 갔어. Both 해서 and 니까 is fine here. The former indicating a simple cause and effect relationship whilst the latter provides your subjective opinion of why you went home. If you were telling the listener for the first time and they had no background of why you went home I would opt for 해서. If the listener already knew you were tired as well, then you could be giving off a “man I’m tired, that’s why I’m going home man”
The second usage of 니까 is to express a realization that follows from completing an action, or observing something. Lets look at some sample sentences
- 직장에 도착하니까 아무도 없었다. When I got to work there was no one there. You can translate it to “having gotten to work I saw that no one was there”.
- 비빔밥을 한번 만들어보니까 생각보다 쉬웠다. After trying to make 비빔밥, it turns out it was easier than I thought. You have done something in the first clause, and now having done that you are now aware of what you have stated in the second clause. You can be specific and translate “After making it for the first time I realised it was easier than I thought” by saying this “비빔밥을 한번 만들어보니까 생각보다 쉽더라” or “…생각보다 쉽다고 깨닫았다” or “…생각보다 쉽다는 걸 깨닫았다”. But unless you want to emphasis the “realise” you can just use the first sentence which already has that connotation built into it.
- 아침에 일어나서 티비를 키고 보니까 박보검이 뉴스 프로그램에서 나왔어. Having turned on and looked at the TV I saw that 박보검 was on a new program. All you did was just watch the TV, and then you became aware of the fact that he was on the news show.
- 신문에서 일기예보를 보니까 내일의 온도가 추워질거래. I watched the weather report on the news and saw that it said the temperature tomorrow will get colder.
In all the examples the speaker has completed an action in the first clause, and based on that action they have become aware of something in the second clause. If there is one English translation that best encapsulates the meaning here I would say it is “having”. “Having gotten to work”, “having tried making it”, “having turned on and seeing the TV”, “having looked at the weather report”.
I want to expand on one more point, which is when 니까 is used with adjectives instead of verbs. Click here for that entry