在复合句中,如果主要动作和背景动作都是延续的或同时发生的,那么主从句的动词都可用过去进行时。
例如: While he was waiting for the bus, he was reading a newspaper.他边等车边看报。 (两个动作都是延续的)
          He was cleaning his car while I was cooking. 他擦车时我在做饭。(两个动作同时进行)
 通常不能用于过去进行时的动词主要有
       agree, is,am,are, believe, belong, care, forget, hate, have(拥有), hear, know, like, love, mean, mind, notice, own, remember, seem, suppose, understand, want, wish等。
例如:误:I was knowing the answer.
        正:I knew the answer. 我知道答案。
     误:I wasn‘t understanding him.
        正:I didn‘t understand him. 我不明白他的意思。