They can't have been doing thier homework.have been 是现在完成时,但

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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?还有They must have been playing in the garden.为什么也用playing,而不用play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

They can't have been doing thier homework.have been 是现在完成时,但 They can't have been doing their homework They can’t have been doing their homework ---doing是什么词性?have They can’t have been doing their homework,能不能改成下面句子 They have been doing their homework He can't have been ill是现在完成时? must have been和can't have been和现在完成时有没有联系啊 can't have been,must have been 结构里是不是不要把have been当现在完成时看,就把它关于现在完成时have been 现在完成时的have been what have they been doing 是什么时态 have been doing sth和现在完成时有区别吗?