这个要分两种情况：
1、must表示“必须,一定”,反意问句用must,如：
You must do it today,mustn't you?
You mustn't do that again,must you?
2.must表示“一定,肯定”推测的时候,和二楼说的一样.要根据情况来定.
如：You must be hungry now ,aren't you?对现在的情况进行推测,用一般现在时.
You must have finished your homework,didn't you?对过去的情况进行推测,用一般过去时.
You must have studied English for three years,haven't you?表示推测的情况有一段的时间状语,反意部分要用现在完成时.