这要看情况而定.
1.如果只是情态动词+动词原形结构,那么一般是根据情态动词来变化.
如:You can play football,can't you?
当然少数情态动词不能照搬否定形式
如:I have to go now,don't
2.如果是情态动词+be doing sth结构,应该是根据be动词的具体形式来变化
如:You must be joking now,aren't you?
He might be sleeping just now,wasn't he?
3.如果是情态动词+have done sth结构,一般也和情态动词无关,应该根据实际情况来判断,这是最难把握的.
如:It must have rained yesterday,didn't it?
其实该句等同:It rained yesterday
You must have ever been there,haven't you?
其实该句等同:You have ever been there
