進行式不定詞用來表達動詞或助動詞後持續性的動作，而這些動詞或助動詞必須接不定詞。進行式不定詞的構成為：
to be + 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。
舉例
- 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.