Both join a reason to a result: attach -아서/어서 to the 아/어 stem, or -(으)니까 to the plain stem. -아서/어서 states a neutral cause and can never carry a past ending or be followed by a command or suggestion; -(으)니까 gives your personal reason and is the one you must use before 하세요 or 합시다.
Examples
- 일이 많아서 어제 못 쉬었어요.
- I couldn't rest yesterday because I had a lot of work.
- 이 일이 재미있어서 이 회사를 선택했어요.
- I chose this company because this work is interesting.
- 시간이 없으니까 택시를 탑시다.
- Since we have no time, let's take a taxi.
The mistake to avoid
Using -아서 before a request (늦어서 서두르세요) - with commands and suggestions it has to be -(으)니까.
Where this fits in
직장과 공부, Korean B1: Talk about your job, workplace, and studies; give reasons and opinions.