1、过去进行时（Past Continuous）
描述一个过去的事件被打断了。To be (过去式) + 现在分词ing。
The boys were riding their bikes, when it started raining.
He was studying when I called. (过去进行时描述之后，后面的动词用简单过去)
2、现在进行时（Present Progressive/ Continuous）
描述一个现在正在进行的事情，眼下正在发生的事。To be(现在式) +现在分词ing.
What is Sally doing?
She is running through the park.
3、未来进行时（Future Continuous）
描述一件将要发生的动作被打断了。will be +现在分词ing
I will be cooking asparagus when you come back this evening.
