1、概述
过去进行时表示过去某一时刻或某段时间正在进行的动作。
The reporter said that the UFO was travelling east to west when he saw it.那位记者说他看见UFO时，它正由东向西飞行。
I don't think Jim saw me; he was just staring into space.我想吉姆没看见我；他当时正在凝视天空。
2、构成
构成过去进行时常由“助动词be的过去时was/were+现在分词”构成。was not常简写为wasn't, were not简略为weren't。
-Hey, look where you are going? 喂，看你上哪了？
-Oh, I'm terribly sorry. I wasn't noticing．噢，实在对不起，我刚才没留神。
It was raining when we left school.我们离开学校的时候正在下雨。
What were you doing when I phoned you last night? 昨晚我给你打电话时，你正在干什么？
3、过去进行时的用法
（1）表示过去某一时刻正在进行的动作。动作发生的特定时间常用一个短语或时间状语从句来表明。
I first met Lisa three years ago. She was working at a radio shop at the time. 我三年前第一次遇见丽莎时。她在一家无线电商店工作。
The last time I saw Jane, she was picking cotton in the fields.我最后一次见简，她正在地里摘棉花。
Tom slipped into the house when no one was looking.汤姆溜进了那所房因为当时没人注意他。
It was raining at 6 o'clock this morning.今早六点钟时正在下雨。
I was drawing a horse when the teacher came in.当老师进来时，我正在画马。
（2）表示过去某段时间正在进行的动作。
Shirley was writing a book about China last year but I don't know whether she has finished it.舍利去年在写一本有关中国的书，但我不知道他是否完成。
They were building a dam last winter.去年冬天他们在建大坝。
I was living in my teacher's house when I was in middle school.上中学时，我住在老师家里。
在含有时间状语从句的复合句中，延续时间较长的动作用过去进行时，另一个动作用一般过去时。如果表示两个延续动作在过去某一时刻同时进行，而不考虑动作的先后长短，则主句和从句的谓语动词都用过去进行时。
My pen dropped on the ground when I was walking in the park.我在公园散步的时候，我的钢笔掉到地上了。
When I entered the room, she was sitting at her desk.我进屋的时候，她正坐在书桌前面。
The students were reading while the teacher was grading their homework.学生们在看书，而老师在批改他们的家庭作业。
（3）用来描写故事发生的背景
在口语或记叙代中，可以用过去进行时表示的持续动作作为背景，以此引出由一般过去时表示的新动作。
It was a dark night. The wind was blowing hard and the rain was falling heavily. A young woman suddenly appeared on the river bank.一个漆黑的夜晚，狂风大作，大雨倾盆，一位年轻的妇女突然出现在河岸上。
Mary was making a dress when she cut her finger.玛丽在缝衣服时，突然扎破了手。
He was just falling asleep when there was a loud knock at the door.他刚要人睡，这时突然听到有人砸门。
（4）代替过去将来时
用于come, go ,leave, start, stay, arrive等表位置转移的动词，以表示过去将要发生的动作。
They wanted to know when we were leaving for Shanghai.他们想知道我们什么时候去上海。
She asked me whether he was starting the next day.她问我他是否第二天就走。
（5）表达褒贬等感情色彩
过去进行时也可以和always，constantly，continually，forever等连用，表示说话人的主观感情，如赞扬、不满、厌烦等。
He was always trying out new ideas.他总是试验一些新的设想。
He was forever complaining about something.他老是怨这怨那。
（6）wonder, hope, think，want 等表示心理的动词用于进行时可以表示婉转的语气
-Excuse me, sir. Would you do me a favor? --打扰了,先生。你能帮我个忙吗？
-Of course. What is it? --当然。什么事？
-I was wondering if you could tell me how to fill out this form. --我一直在想你是否能告诉我怎样填这张表。
-What were you wanting?你要什么？
- I was hoping you would help me with the work.我希望你能帮我做这项工作。
4、一般过去时与过去进行时用法比较
一般过去时表示在过去某个时间发生的动作或存在的状态（强调一次性动作）；而过去进行时则表示在过去某一时刻或某段时间正在进行的动作（强调反复的动作）。
Mary wrote a letter to her friend last night.玛丽昨晚给她的朋友写了封信。（写完了）
Mary was writing a letter to her friend last night.玛丽昨晚一直在给她的朋友写信。（不一定写完）
We built a bridge last winter.去年冬天我们修了一座桥。（建成了）
We were building a bridge last winter.去年冬于我们在修一座桥。（一直在修，不涉及结果）
As she was reading the newspaper, Granny fell asleep.奶奶读报时睡着了。