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.
however 也是副词,剧中成分相当于连词.可以放在句首或句中,后面一定要用逗号.表转折.放句首时,与前面的句子是转折的连接.放句首时,前后用逗号隔开
如：I wanted to buy a book, however, i do not have enough money.