hey.. dale, honestly i never thought about this question after i finished my elementary school.. because as a chinese, its so natural to say both of these words without thinking too much,well, your question makes me wonder again.. does there any rules exist there to make them differ from each other? haha well, literally both of them does mean "but" in english, But "但是" may sound stronger than" 可是”， and " 可是“could be used alone, and normally when people use 但是,, we connected 虽然 with 但是 together when making up a new sentence..i hope this could help you figure out this confusion..

这真是一个有趣的问题，我估计每个中国人都不会自己考虑这个问题。 其实这个可是和但是并没有实际的区别，可是经常用于口语，而但是经常用于书面语。 呵呵，看来您学习中文挺认真的。 It is an interesting question. In my opinion, most of chinese people never consider this problem carefully. In fact ,there are not many distinctions between "ke shi" and "dan shi" except " ke shi" is used more in oral but "dan shi" is used more in writting i think you learn chinese very seriously. not everyone could find it.

Maybe there is a tinny difference between keshi and danshi,when they both represent "but". Actually in spoken Chinese,we don't have so many rules using which.The both can express your feelings well and nobody will misunderstand it.Even I can't distinguish them very clearly...LoL But pay attention!Only when they mean "But" I think you are a very careful learner..cheer up!

可是; 但是, 对中国人来说，都是转折的意思,没有太大的差别,一般不会造成误解; 可是,(这里用可是比但是好)如果非要找出不同,我认为: 1.语气强弱 2.更重要的是:可是=通常应该是,但是不是(abnormal);可是=means you can or may have other choice sometimes 但是=完全对立(it should be,but absolutely not)---yes or not ,when you don't want to accept or agree at all, use but(但是) Jurion+ivy's examples are very good.

i think if use ke shi(可是),it means you are not sure. when u use dan shi(但是)，it means you are sure or you want to be a agreed。 i think so.