Which sentence is right- (으)니까 or 아/어서 here?
To say “But because the exam is soon I have to study.” Which is correct?
하지만 시험이 곧 있으니까 공부해야 돼요
하지만 시험이 곧 있어서 공부해야 돼요

Question

Which sentence is right- (으)니까 or 아/어서 here?
To say “But because the exam is soon I have to study.” Which is correct? 
하지만 시험이 곧 있으니까 공부해야 돼요
하지만 시험이 곧 있어서 공부해야 돼요

Answer

You can use both form here.  Both '으니까' and '어서' is used for showing the reason of the following action.

But sometimes they cannot be used in certain sentences.
For example,  '어서' cannot be used before imperative sentences while you can use '으니까' before them.

e.g.  시간이 없어서 서두르세요. (X)
         시간이 없으니까 서두르세요. (O)

Here are some idiomatic expressions using them.

e.g. 만나니까 반가워요. (X)
       만나서 반가워요. (O)

도와주시니까 감사합니다. (X)
    도와주셔서 감사합니다. (O)

* Hope it helps and please follow me at koreanclass.canada on Instagram!