标志：was / were + 动词的现在分词

1.  表示过去某一时刻或一段时间正在进行的动作，过去进行时中常用的时间状语有the whole morning, all day yesterday, from January to March last year等。如：

I was having a talk with Lucy at that time.

They were watching TV at home last night.

2.  表示过去反复出现或习惯性的动作，往往包含说话者赞扬、责备、厌恶等情绪，通常与always, constantly, continually, forever等频度副词连用。如：

My brother was always losing his keys.

3.  表示按计划、安排过去某时刻将要发生的动作，仅适用于部分趋向动词（如go, come, leave, start, arrive等）。如：

He said they were leaving for Beijing this afternoon.

4.  过去进行时有一个主要用法就是描述一件事情发生的背景（一个长动作延续的时候，另一个短动作发生）：

Granny fell asleep when she was reading.

It was raining when they left the station.