被动语态不定词
被动式不定词是用在某些使用被动语态的句构上。尤其常用在助动词（may丶should丶could 等）后面来表达「什麽是可能的」丶「该如何做才正确」，尤其常见。被动不定词的构成为：
to be + 过去分词
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。
举例
- I am expecting to be given a pay-rise next month.
- She is hoping to be elected president.
- The carpet needs to be washed.
- These doors should be shut at night.
- This window may be opened but that one must stay closed.
- Alice could be given a prize for her artwork.