Also -(으)나 can be used in the conditional tense, like 내일은 맑겠으나 모레는 비가 오겠습니다. I used -ㅂ니다 here as you see. What does this mean? -(으)나 is used in formal writing. We use 근데/-ㄴ데(but/and) instead of 그러나, 하지만(these mean only "but" and are used in formal writing). 어제 거기 갔는데, 너 없던데? I went there yesterday, but you weren't there, right? 이건 그냥 하는 말인데, 내일 바빠? I'm not saying this seriously(or "Don't take this seriously"), are you busy tomorrow? -니까 so, and, because 나 지금 가고 있으니까 조금만 기다려. I'm going there, wait a little. 내가 너무 늦게 왔으니까 밥 쏠게. Because I came here too late, I'll pick up the tab for our meal. -서 so 시간이 없어서 못 가겠는데. I have no time, so I can't go there. We combine the stem of the verb with the conjunction(the stem is deleted) in order to make what we say sound smooth. See "맑겠다 + 그러나". -다 is a basic ending, delete it. It became "맑겠- + 그러나". 그러- is the stem of the verb, "그러하다 be like that", delete it. It became "맑겠- + -나". 맑겠- ends with "ㅆ", add 으 in front of -나. It became 맑겠으나 after all. I believe you can work on the rest, "갔다 + 그런데(그러한데)", ""가고 있다 + 그러니까", "왔다 + 그러니까", "없다 + 그래서(그러해서)". :)

~으나 means "but" mainly, ~으나 comtain past meaning. i liked apple.but i didn't eat apple. 나는 사과를 좋아했으나(했지만) 나는 사과를 먹지 않았다. _______________________________________좋아했다. 그러나(=하지만) i like apple. but i didn't eat that. 나는 사과를 좋아한다. 그러나 나는 그것을 먹지 않았다. 했다 = past form / 한다=presnt form ~니까 = hence, That is why~~ ~서 express causal relationship..(~해서, 그래서..) (~로서 means qualification ) my idea isn't a little right:-p