They can't have been doing thier homework.have been 是现在完成时,但是后面do为什么要变成doing?They can't have been doing thier homework.这句话 have been 是现在完成时,但是后面do为什么要变成doing?为什么不用do呢?

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They can't have been doing thier homework.have been 是现在完成时,但是后面do为什么要变成doing?They can't have been doing thier homework.这句话 have been 是现在完成时,但是后面do为什么要变成doing?为什么不用do呢?

They can't have been doing thier homework.have been 是现在完成时,但是后面do为什么要变成doing?They can't have been doing thier homework.这句话 have been 是现在完成时,但是后面do为什么要变成doing?为什么不用do呢?
They can't have been doing thier homework.have been 是现在完成时,但是后面do为什么要变成doing?
They can't have been doing thier homework.这句话 have been 是现在完成时,但是后面do为什么要变成doing?为什么不用do呢?还有They must have been playing in the garden.为什么也用playing,而不用play呢,

They can't have been doing thier homework.have been 是现在完成时,但是后面do为什么要变成doing?They can't have been doing thier homework.这句话 have been 是现在完成时,但是后面do为什么要变成doing?为什么不用do呢?
现在完成时结构 have/has+P.P.
现在完成进行时结构 have/has +been+doing
此句在现在完成进行时 前加上情态动词,can't have been doing 表示一种语气较肯定的推测.意为那时不可能在做某事

LZ这叫现在完成进行时

can't have been doing 是情态动词表示推测，而且是对过去正在发生的动作表示推测，意为那时不可能在做某事。must have been doing表示那时一定在做某事，是对过去正在发生的肯定推测。这个语法点在新概念一册129课130课。

首先have been 后面绝对不可能接do 原型，要么是have done! have been doing 是现在完成进行时，但依据你说的句子，句中另有must ,所以应该是must have句型，表推测(此处表示对现在进行的动作推测)，所以最后是have been doing

这不是现在完成时，是现在完成进行时

They can't have been doing thier homework.
他们不可能一直还在做作业

表示对已经发生的事作以否定的推测，要用
can't have done
你你可能看过这部电影。 You can't have seen the film

如果过去发生的某个动作现在可能还在持续， 用：
can't h...

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They can't have been doing thier homework.
他们不可能一直还在做作业

表示对已经发生的事作以否定的推测，要用
can't have done
你你可能看过这部电影。 You can't have seen the film

如果过去发生的某个动作现在可能还在持续， 用：
can't have been doing
他们不可能还在等。 They can't have been waiting

注意： 如果是肯定推测， 就不可用can, 而用may, must(最有把握时用）
你可能看过这部电影。 You may have seen the film. (不用can)
你一定看过这部电影。 You must have seen the film.

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