1. ~을/ㄹ 수가 있/없,..= The "가", is it used for written or spoken ? What kind of situation ? Formal ? Informal ? 나는 김치를 먹을 수 있다. 도저히 먹을 수(가) 없다. The "가" is used for any situations(you can use it both), we sometimes use it in negative sentence, but most time, we omit it(because we want to say short sentence). 2. ~기에 = As far as i know. This one meaning is "because". Then when i can use it ? How about another "Because". ~니까, 어/아서? Is it interchangeable? 그러기에 내가 가지 말라고 말한 겁니다.( that's the reason why I told you not to go) = 그러니까 내가 가지 말라고 말한 겁니다. = 그래서 내가 가지 말라고 말한 겁니다. In this case, you can use all... but we use 그러니까, 그래서 more than 그러기에. for example, 아침 밥 먹으니까 배부르다. / 아침 밥 먹어서 배부르다. (I'm full because I had breakfast. )

Wow.That's a great information Mr. Nicholas. "어/아서 is more frequently used" Because i get some informations from my friend (well actually both of us never go to Korea so we don't know the real truth). She told me that "~으/이니까" is used for any type of sentences. Then "어/아서" is just for declarative and interrogative. That's why I told "going to take ~으/이니까". I am still very very very beginner to Korean Language. I'm sorry for Korean if i make alot of mistakes.^-^

What do you mean with that you're "going to take ~으/이니까"? They are not always interchangable. 어/아서 is more frequently used, so you should learn about that first in my opinion.

Thank you very much for the information.Mr.Jon. I think i'm going to take "~으/이니까". It is more universal.

I think you should listen to this lesson for more information about the differences between (으) 니까 and 어/아서. http://www.talktomeinkorean.com/lessons/level-5-lesson-14/