陈述句含有情态动词must有两种情况：
must表示“必须”,反意疑问句部分为mustn’t…? / needn’t…?
He must study hard at English, mustn’t he? / needn’t he?
You must go home now, needn’t you? / mustn’t you?
We mustn’t be late, must we ?
Must表示推测：“一定,肯定” 反意疑问句部分与must后面的动词呼应
You must be joking, aren’t you?
He must be ill, isn’t he ?
注意：用must对过去的动作推测时,反意疑问句部分的助动词用did或have, 而对过去的状态推测,反意疑问句部分的be动词用was:
She must have finished her work, hasn’t she ? / didn’t she ?
Jack must have arrived here yesterday, didn’t he ?
He must have been a policeman, wasn’t he ?