基本信息
when， as， while (从属连词) when既可引导持续性动作，又可引导短暂性动作。它可以表示主句的动作和从句的动作同时发生，或从句的动作发生在主句动作之前 when i was a boy, i used to go to the seashore on sundays.(同时) when the lesson was over, we began our writing.(从句动作在前) as表示“一边……一边……”，引导持续性动作，侧重主句和从句的动作同时发生 he hurried home, looking behind as he went. i saw your sister as she was getting on the bus yesterday. he sang as he walked.
