用when,while,引导的时间状语从句怎样辨别主句和从句?

用when,while,引导的时间状语从句怎样辨别主句和从句?

when和while的区别和用法
when和while引导的时间状语从句与这一时态连用.区别when和while的用法成了的难点.（一）、when,while都有“当……时候”的意思.when既可表示某一点时间,也可以表示某一段时间 .在when引导的时间状语从句中,其谓语动词可以是延续性的,也可以是非延续性的,可与主句中的谓语动词同时发生,也可在其后发生.例如：1、I was just reading a book when she came into my room.她走进我房间时,我正在看书.2、Were you writing when the teacher came in?老师进来的时候,你在写信吗?3、When he was a child he was always trying out new ideas.他小时候就常常试验一些新的设想.（二）、while只能表示某一段时间,不能表示某一点时间.在while引导的时间状语从句中,其谓语动词只能是延续性的,而且也只能与主句中的谓语动词同时发生或存在.例如：1、While Jim was mending his bike,Lin Tao came to see him.正当吉姆修自行车时,林涛来看他.2、You can’t do your homework while you’re watching TV.你不能一边看电视一边做家庭作业.（三）、另外,when和while的区别还在于：while引导的时间状语从句多用进行时态,而when引导的时间状语从句多用一般时态.例如：1、While they were talking ,the bell rang.正在他们谈话的时候,上课铃响了.2、I was doing my homework when my mother came back home yesterday evening.昨天晚上妈妈回家的时候,我正在做家庭作业.注意：1) 如主句谓语动词表示的是短暂动作,从句时间状语指一段时间,when while as 三者可通用.
例如：Peter came in while /as I was watching TV.当Peter进来的时候我正在看电视.
I was watching TV when Peter came in.当Peter进来时我正在看电视.2) as 和 when 可与非延续性动词连用,而 while 只能与延续性动词连用.例如：He came in when/ as I was going to bed.我正要上床睡觉,他进来了.3) as 和 while可用来指一段时间,但 as 强调主句与从句表示的动作同时发生,while强调主句表示的动作是在 while 从句的动作延续中发生.例如：As my mother sang those old songs,tears ran down her cheeks.当我妈妈唱起那些老歌时,眼泪顺着她的脸颊流了下来.I came in while he was reading an interesting book in his room.