进行式不定词用来表达动词或助动词后持续性的动作，而这些动词或助动词必须接不定词。进行式不定词的构成为：
to be + 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。
举例
- I'd really like to be swimming in a nice cool pool right now.
- I happened to be waiting for the bus when the accident happened.
- It must be nice to be going to a wedding overseas.
- You must be joking!
- We should be hurrying. We're late!
- You'd better be working on your report when I get there.
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