1 you must be tired,aren't you?2 you must go home right now,needn't you?3 you mustn't walk on grass,must't you?为什么都含must,反意疑问句都不一样~

“must”在反意疑问句中的用法
1.must作“必须”解释,mustn't作“不允许”解释时
They must clean the floor,mustn't they/needn't they?
他们必须擦地板,是不是?
We must stay at home,mustn't we?
我们必须留在家里,是不是?
We mustn't be late,must/may we?
我们不许迟到,是不是?
2.must表示推测时：
They must be playing basketball,mustn't they?
他们肯定在打篮球,是不是?
逗号前的陈述句中的谓语部分含有“推测”性的must 时,先把must去掉再看后面的动词该用什么时态和形式.
Her mother must be a teacher,isn't she?
must去掉,后面的动词应该是is .
She must have read the novel,hasn’/didn't she?must去掉 后面的动词应该是has read 或 read .因为用must对现在完成时和一般过去时的推测都是must have done 形式.句子中有时间状语时比较容易区别.
He must have come now ,hasn't he?
He must have come last week,didn't he?
但是：You must(必须) work hard next time ,musn't you?
You must(必要) go home right now ,needn't you?