可是 / 但是【kěshì / dànshì】

可是 and 但是 both mean "but". They open the second clause.

可是 or 但是

可是 is lighter and belongs to speech. 但是 is a little firmer and turns up more in writing. In conversation they swap freely, and neither is wrong where the other would go.

A clause, not a phrase

What follows has to be a clause with its own predicate. The subject can drop when it's the same as in the first clause.

• × 我喜欢咖啡但是茶 → 我喜欢咖啡，但是不喜欢茶

With 虽然

虽然 "although" in the first clause pairs with 但是 or 可是 in the second. English drops one of the two; Chinese keeps both, and it isn't redundant.

不过 is a softer "but", closer to "though".