2)They must have arrived by now,haven't they?为什么是haven't they?谁帮我讲一下什么时候用didn’t 什么时候用haven’t

Question

2)They must have arrived by now,haven't they?为什么是haven't they?
谁帮我讲一下什么时候用didn’t 什么时候用haven’t

Answer 1

情态动词+动词完成时,即情态动词+have+done.表示对过去的行为或动作进行推测,评论或者判断.
1.must have done.
表示对过去某事的肯定猜测,其否定或疑问形式都用can/could来表示.
例如：since the road is wet,it must have rained last night.
2；当然对现在发生或者将来发生的事情,要用must do表示猜测,否定为can’t do
he must understand that we mean business.
you must be hungry after a long walk.
反意疑问句中含有Must的情况主要有以下几种：
1）作为情态动词表“必须”,这时反意疑问句直接用mustn'/needn't
2)当must表示推测时又分以下几种情况：
a：对现在事实的推测,反意疑问句与must后面的动词呼应,如:
You must be joking,aren't you
b:对过去事实的推测,表示动作的时候用did 当助动词,表示状态时用was,如：
Mr Green must have been punished for his being rude at the meeting,didn‘t he (格林先生一定因为昨天在会上鲁莽的行为被处罚了,是吗?） 被处罚表示一个动作
She must have been a policeman ,wasn't she （她过去一定是个警察,是吗?） 是警察表示一种状态
对现在情况的推测,问句部分用主动词（do,be)一般现在时的适当形式.若是现在进行时,问句部分用现在进行时的适当形式表示.若是there be结构,问句用isn't/aren't there.如：
1)He must be there,isn't he?
2)He must have a big family,doesn't he?
3)He must be waiting outside,isn't he?
4)There must be some students in the room,aren't there?
对已发生的过去情况的推测,若陈述句谓语部分有“must have done”,而且有表示过去的时间状语,问句部分用didn't；若没有表示过去的时间状语,问句部分用haven't或hasn't.如：
1)They must have gone there last night,didn't they?
2)They must have arrived by now,haven't they?（根据by now来判断）
3)They must have been to the Great Wall,haven't they?
若是被动,应按被动结构来处理.如：
1)The room must have been cleaned yesterday,wasn't it?
2)The room must have been cleaned,hasn't it?
若句中有表示过去完成时的时间状语,问句部分应用hadn't.如:
They must have learnt 5000 English words by the end of last term,hadn't they?
(本题中must表推测,如果将它去掉,还原为真实句就是They had learnt 5000 English words by the end of last term.因此,反意问句是hadn't）