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。
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)
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)
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。