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?
问题描述：
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,
答
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