过去进行时和一般过去时的比较

1）过去进行时和一般过去时的基本差别是，过去进行时表示一个正在进行的动作，而一般过去时表示一个已完成的动作。试比较下面的句子：
I was writing an essay. 我在写一篇论文。（可能还没写完）
I wrote an essay yesterday. 我昨天写了一篇论文。（已写完）

They were building a skyscraper. 他们在建一座摩天大楼。（在修建）
They built a skyscraper. 他们盖了一座摩天大楼。（已盖好）

She was knitting a sweater for Tom. 她在给汤姆织一件毛衣。
She knitted a sweater for Tom. 她给汤姆织了一件毛衣。

He was doing his homework in the afternoon. 下午他在做作业。
He did his homework in the afternoon. 下午他把作业做完了。

2）有些动词本身并不表示动作完成，如 rain, snow, cough, wear, feel, wait, stay, work等，这时用两种时态意思上差别不大：
It snowed all night.       下了一整夜的雪。
It was snowing all night.

She wore a blue coat.      她穿着一件蓝外套。
She was wearing a blue coat.

He felt a bit uneasy.        他感到有点不自在。
He was feeling a bit uneasy.

We stayed in a hotel.       我们住在旅馆里。
We were staying in a hotel.

3）用进行时态有时有感情色彩，特别是带有always, forever, constantly这类状语时，如：
He was always ringing up. 他总是给我打电话。（厌烦）
She was always working. 她总是不停地干活。（赞扬）
The old lady was forever grumbling. 这老太太老是嘀嘀咕咕。（厌烦）
He was constantly changing his mind. 他总是改变主意。（不以为然）

4）有时用进行时表示现在的想法，可以显得客气一点，如：
I was wondering whether you could give me some advice. 不知你能否帮我出点主意。
I was thinking maybe he could go by taxi. 我在想或许他可以坐的士去。
I was hoping you could reconsider our proposal. 我希望你能再考虑一下我们的建议。