时间状语从句
1.when, while,as引导的时间状语从句的主、从句动作发生的顺序及主、从句的时态要求。
(1)主、从句动作发生的顺序
when与as引导的时间状语从句，主、从句谓语动词可以同时发生也可以先后发生; while引导的时间状语从句，主、从句谓语动词同时发生或主句动作发生在从句动作进行的过程中。
When he had finished his homework. he took a short rest
当他完成作业后，他稍微休息了一会儿。(从句动作先发生)
I got to the airport, the guests had left.
当我赶到机场时，客人们已经离开了。(从句动作后发生)
While we were working in the field, it suddenly began to rain
当我们正在地里劳动时，天忽然下起雨来。
(2)主、从句的时态要求
如果主句的动作发生于从句的动作进行的过程中，从句常用进行时。
Someone broke into the house when/while/as she was sleeping
在她睡觉时有人闯人屋里。