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,