to have been + 現在分詞
如同現在式不定詞，有些時候必須用上不帶 to 的被動語態不定詞，因此 to 會被省略掉。這種情況會發生在多數情態助動詞後，以及用在其他使用零不定詞的地方。
- The woman seemed to have been crying.
- He pretended to have been painting all day.
- I would have preferred to have been sleeping all afternoon.
- You must have been waiting for hours!
- They might have been talking before you came in.
- You should have been studying for your test.