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.