but：I’ve received a gift from him, but I’m not going to accept it.

yet：He worked hard, yet he failed. 他工作很努力，可是他失败了。

while：Carbon is an element, while carbon dioxide is a compound(化合物).

or：Turn the heat down or your cake will burn.

or else：I had to defend myself or else he’d have killed me.

otherwise：It’s lucky that he bounded away, otherwise he would have been hurt.