when 引导的状语从句：
when从句的谓语动词可以在主句谓语动作之前、之后或同时发生。
例句：When he had finished his homework，he took a short rest.(finished先发生)当他完成作业后，他休息了一会儿。
when从句的谓语动词可以在主句谓语动作之前、之后或同时发生。
例句：When he had finished his homework，he took a short rest.(finished先发生)当他完成作业后，他休息了一会儿。
