(1) 过去进行时通常表示一个过去正在进行的动作；而一般过去时则通常表示一个过去已经完成的动作。如：
I was reading the newspaper last night. 我昨晚在看书。(不一定看完)
I read the newspaper last night. 我昨晚看了报纸。(已经看完)
He was building a model ship out of wood. 他在那里修一座桥。(不一定建成)
He built a model ship out of wood. 他用木头造了个模型船。(已建成)
(2) 少数动词如rain, snow, cough, wear, feel, work等的一般过去时并不表示动作完成，这时用两种时态都可以，且含义区别不大。如：
It snowed / was snowing all day yesterday. 昨天下了一天的雪。
She didn't feel / wasn't feeling well that day. 那天她感到不太舒服。
The child coughed / was coughing all night. 这孩子咳嗽了一整夜。