过去进行时
1）概念：表示过去某时正在进行的状态或动作。
2）过去进行时的主要用法是描述一件事发生的背景；一个长动作发生的时候，另一个短动作发生。
3) 常用的时间状语
this morning, the whole morning, all day yesterday, from nine to ten last evening, when, while
My brother fell while he was riding his bicycle and hurt himself.
It was raining when they left the station.
When I got to the top of the mountain, the sun was shining.
