They must have done their homework yesterday,____?这个句子的反意疑问句为什么是didn't they呢?

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They must have done their homework yesterday,____?这个句子的反意疑问句为什么是didn't they呢?
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They must have done their homework yesterday,____?这个句子的反意疑问句为什么是didn't they呢?
They must have done their homework yesterday,____?
这个句子的反意疑问句为什么是didn't they呢?

They must have done their homework yesterday,____?这个句子的反意疑问句为什么是didn't they呢?
must have done 表示的是对过去动作进行推测,其反义疑问句的构成有两种:didn't .../wasn't(weren't) .../haven't(hasn't)...这两种形式的选择取决于句子中的时间状语.比较一种简单的方法是:不把它看成是推测,就把它看成是一种事实.
比如:He must have been to Beijing,hasn't he?
You must have been late for school yesterday,wasn't you?
换句话说,用过去时还是用现在完成时来进行反问的区别,等同于过去时和现在完成时的区别.
又如:It must have rained last night,didn't it?

must 表示推测时,反问部分不用must ,要按照陈述句原来的事时态结构反问

must have done表示对过去的肯定推测 因此用did`t

they must必须这些,didn't不,they不变,所以是didn't they。明白???