They can't have been doing thier homework.have been 是現在完成時,但

題目：

They can't have been doing thier homework.have been 是現在完成時,但是後面do爲什麼要變成doing?
They can't have been doing thier homework.這句話 have been 是現在完成時,但是後面do爲什麼要變成doing?爲什麼不用do呢?還有They must have been playing in the garden.爲什麼也用playing,而不用play呢,

解答：

現在完成時結構 have/has+P.P.
現在完成進行時結構 have/has +been+doing
此句在現在完成進行時 前加上情態動詞,can't have been doing 表示一種語氣較肯定的推測.意爲那時不可能在做某事