过去时主要是指过去发生的动作或存在的状态
而过去进行时表示过去某阶段正在进行的动作,通常有时间暗示.
I did my homework yesterday .
I was doing my homework this time yesterday .While the pupils were doing classwork,their teacher returned.这个句子都没有时间暗示，那为什麼要用Past continue tense呢？这里是属于时间状语从句中的过去进行时，当小学生们当时正在做功课的时候，老师返回来了。while/when/as引导的时间状语从句如果是长时间动词，主句是短时间动词，长时间动作用过去进行时，短动作用一般过去时。While /When /As she was walking on the street, it began to rain.还有这样的句子， I was cooking whensomeoneknocked at the door.