but 是连词，连接两个并列句，中间用逗号隔开。表示转折

如： I wanted to buy a book, but I don't have enough money.

though 是副词，但在句中的成分相当于连词。引导从句，不能与but 连用，可以放句首或者句中。意思是尽管

如：Though it is so hot I have to go outside.

I have to go outside though it is so hot.