to have been + 现在分词
如同现在式不定词，有些时候必须用上不带 to 的被动语态不定词，因此 to 会被省略掉。这种情况会发生在多数情态助动词后，以及用在其他使用零不定词的地方。
- 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.