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。
