While they all make a contradictory or contrasting sentence, 그런데 is a little different in usage and meaning. - 그러나, 그렇지만, 하지만 = "(That may be so) but ..." - i.e. it introduces a phrase squarely opposing the preceding one. - 그런데 = "That is so" and /but / in addition ... - 그런데 relates the two clauses in subtle ways. It has an underlying tone of "Here's a new thing about it, which is ...", often introducing something new or unexpected. 그러나 sounds a little too literary to be used in conversation. 그렇지만 and 하지만 are almost the same words. There is no need to distinguish between them. Also note that these words generally come at the beginning a clause or sentence (so their use is limited). There are connective phrases -지만 and -는데 with a similar sense (그렇지만/하지만 and 그런데 are like special cases of these), which connect two clauses. For example, A: 오늘 학교에 늦었어. 그렇지만/하지만/그런데 선생님도 늦으셔서 괜찮았어. (I was late for school today. But the teacher was late too, so I lucked out.) B: 오늘 학교에 늦었지만/늦었는데 선생님도 늦으셔서 괜찮았어. * The two facts are loosely opposing (more or less parallel), so both -지만 and -ㄴ데 may be used. A: 우리는 가난하다. 그러나/하지만/그렇지만 희망이 있다. (We are poor, but we have hope) B: 우리는 가난하지만 희망이 있다. (We are poor, but we have hope) * When contrasting two thing for great effect as here, 그러나 and 그렇지만/하지만 work better.