1.过去进行时强调动作在过去某时刻正在进行或持续，而一般过去时表示动作的完成。如： He was writing his composition last night.
他昨晚在写作文。(不一定写完)

He wrote his composition last night.

他昨晚写了一篇作文。(已经写完)

2.表示过去的状态、感觉及心理活动的静态动词(如be, like, love, hate, fear, own, hear, see, know, want, notice)可用于一般过去时，但通常不用于进行时。如：
I hated it when a man spoke with his mouth full of food.

我讨厌人们说话时口里含着食物。

3.一般过去时与always, constantly, forever, continually等连用，表示过去经常性、习惯性的动作；而过去进行时与always, constantly, forever, continually等连用，表示动作的重复，常带有感情色彩。如：
He always got up at six.

他过去总是6点起床。

He was always thinking of his work.

他总是一心想着工作。

4.有时过去进行时可以用来替换一般过去时，但一般过去时表示主语的行为是经过认真考虑的；而过去进行时表示一种较随便或没有进行仔细考虑的行为。如：
I thought that he would agree with us.

我认为他会同意我们的观点。

I was thinking of persuading him to follow my advice.

我想要说服他接受我们的建议。

例句对比：
1.I read a book yesterday. (书已看完)
I was reading a book yesterday.(书尚未看完)
2. The guests arrived.(客人已到)
The guests were arriving.(客人陆续到达)
3.He woke from a dream. (表示全醒)
He was waking from a dream.(表示初醒)
4.The old man died.(已死)
The old man was dying.(要死)
6.John told me about it.(告诉我了,我都知道了)
John was telling me about it.(跟我谈起过,我想了解更多的事情)
7. They persuaded me to go along with them.(已经说服)
They were persuading me to go along with them.(还在劝说)
8. The wind blew hard all night.(强调事实)
The wind was blowing hard all night.(强调风刮个不停)
9. He knocked at the door.(强调一次性)
He was knocking at the door.(强调多次性)