这里为什么用完成时.the exam must have been very difficult.her husband

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这里为什么用完成时.
the exam must have been very difficult.
her husband had been killed in a car crash.
完成时我记得是强调对现在或者过去的影响,但是这两个句子里体现不出来啊,帮我分析下这两个句子,

must have done/been 表示的是对发生在过去的事情的一个肯定猜测,
比如,他一定是把蛋糕都吃了.He must have finished all of the cakes.
否定猜测是can't have done/been,比如他不可能把这些蛋糕都吃了 He can't have eaten all of the cakes!
跟这个类似的还有,should (not) have done/been,might (not) have done/been need (not) have done/been,只有must 不能直接说must not have done/been 用can not have done/been
第二句,我回答过其他关于过去完成时的用法,过去完成时有时候特指时间点的位置,表示比过去式早一个点,比如我给你改成," When I broke into the house,her husband had been killed." 当我破门而入了,她丈夫已经被杀了.表示在进去之前就死掉了.有先后顺序.也跟你说的"对过去的影响" 不冲突啊,如果他在我进门后死的,表示我这个警察办事不利,但是我冲进去前就死了,那是没办法的事.

The bulidings have been designed by Kurt Gunter为什么用完成时 He has often been there.这里为什么要用完成时 I am very glad to have received the letter中to后面为什么用完成时?to后是不初中英语为什么这里要用完成时 It must have been they who picked the flowers.这里的强调句的must ha what could have been a very difficult case for the defense.在 --The old lady has been sad since her husband died. l think the g____ exam is very difficult the clothes must have been washed by her just now,___A.didn' 但为什么This is my first time to have been here.里要用完成时?谢谢~~ The question___us is a very difficult one.为什么填before?这里befor 1.She’s very because her husband has been out of work for mo