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选修7《Grammar》新课标PPT课件优质课下载
I've been very busy since I came back from holiday.
状语从句
状语从句在主从复合句中修饰主句中的动词，形容词或副词，按意义可分为时间、地点、原因、目的、结果、条件、方式、比较、让步等状语从句。从句可放在句首，也可放在句末。放在句首时，从句后面常用一个逗号，放在句末时，从句前一般不用逗号。本模块着重讲解时间状语从句。
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时间状语从句：
引导时间状语从句的从属连词有：when, while, as, whenever, before, after, since, till, until, as soon as, once,the moment, immediately, no sooner…than, hardly…when, by the time, every time, last time…等。
（1）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。如：
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.
while指的是“在某一时间里”，“在……期间”，从句里的动作必须是持续性的，它也强调主句和从句动作的同时发生，往往侧重主句和从句动作的对比。
如：While he was in London, he studied music.
While we were watching TV, he was writing a composition.
当when, as, while表示“在…一段时间里”，主从句的动作同时发生时，可以换用。如：
While (When/As) we were discussing, Mr. Smith came in.
when 可表示“如果”。如：
Don’t be afraid of asking for help_____ it is needed. (2003全国）
A. Unless B. since
C. although D. when
while可表示“虽然，尽管”。如：
1)____ modeling business is by no means easy to get into, the good model will always be in demand.(2004浙江）