可是 / 但是【kěshì / dànshì】

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

Which one

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

Both open a clause, not a phrase

wǒ我xǐ huan喜欢kā fēi咖啡dàn shì但是chá茶 is wrong. What follows has to be a full clause with its own subject and predicate.

With 虽然

Chinese keeps both halves of the pair, where English drops one. 虽然…但是… is normal and not redundant.

Writing "Although it was expensive, but I bought it" in English is an error; writing it in Chinese is the standard form.

不过 is the softest of the three