Grammar Tips -(으)니까 vs -아서/어서

How is '-(으)니까' different from '-아서/어서'?
It can be used both when it's meaning is 'cause' or 'reason'. When it should be used '-(으)니까 ' and '-아서/어서?'
First, only '-(으)니까' can be used to mean suggestions or commands. These sentences start 'Let's..., Shall..., Do... .'
For example, in the previous conversation,
"돈이 생겼으니까 어디 가자. Let's go somewhere because we have money." "날씨가 추우니까 부산에 가는 게 어때? It's cold these days, so why don't we go to Busan?" "시간이 없으니까 지금 당장 가자. We don't have much time, so let's go right now." 

You can't exchange these sentences '-아서/어서' to -(으)니까. By the way, you have to use '-(으)니까' also when it means suggestion or command although the sentence looks like a declarative sentence. "돈이 생겼으니까 어디 가요.", "날씨가 추우니까 부산에 가요.", "시간이 없으니까 지금 당장 가."
Second, we often use '-아서/어서' to explain other reasons except for suggestions or commands. For example, "로또에 당첨돼서 돈 좀 쓰려고 해. I won a Lotto, so I wanted to spend some money." "나도 머리가 아파서 어디 가고 싶었어. I wanted to go somewhere because I had a headache." It's a bit awkward to use '-(으)니까' at times like this. 

You should be careful when you use '-(으)니까' in declarative sentences. Because It sounds rude.

For example, when it combines two sentences like "늦었다 I am late + 죄송합니다. So, I'm sorry.", "도와주셨다 You helped me + 감사합니다. So, thank you.", you should use grammar '-아서/어서'. It means "I'm sorry, I am late", "Thank you for your help."
If you use '-(으)니까' these sentences, it will make the listener feel bad even if you say 'sorry', and 'thank you.'

Conclusion:
In the sentences including the meaning "suggestions" or "commands," it is used "-(으)니까". For the rest, it is used "-아서/어서"
What do you think? Now you can tell the difference between the two, right?
For more information, see the link below.
Bye.