1、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;while和as从句的谓语动作必须是和主句谓语动作同时发生。 1.从句动作在主句动作前发生,只用 when。 ①When 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。 ②When I got to the airport,the guests had left.(got to后发生)当我赶到飞机场时,客人们已经离开了。 2.从句动作和主句动作同时发生,且从句动作为延续性动词时,when,while,as
2、都可使用。3.①When /While /As we were dancing,a stranger came in.(dance为延续性动词)当我们跳舞时,一位陌生人走了进来。 ②When /While /As she was making a phonecall,I was writing a letter.(make为延续性动词)当她在打电话时,我正在写信。 3.当主句、从句动作同时进行,从句动作的时间概念淡化,而主要表示主句动作发生的背景或条件时,只能用 as。这时,as常表示“随着……”;“一边……,一边……”之意。 ① As the time went on,the
3、 weather got worse.(as表示“随着……”之意) ② The atmosphere gets thinner and thinner as the height increases.随着高度的增加,大气越来越稀薄。 ③As years go by,China is getting stronger and richer.随着时间一年一年过去,中国变得越来越富强了。 ④The little girls sang as they went句子如下: That day, I was doing my homework ___ my younger brother
4、 was watching TV. Suddenly the doorbell rang. A.when B.while C.before D.after when 和while都可以表示“两个动作同时发生”的含义, 此时它们的不同之处在于while 有表示“对比”的含义, 如题目中一人在做作业,一人在看电视,强调两人做不同的事情,有对比的含义 when和while都可以用在多种行为或情况同时发生。具体情况有下面四种: 1.我们经常用when和while引导时间状语从句引出一个大的背景,而在这个背景之下发生发生了主句所描述的行为。 i.e.Somebody bro
5、ke into the house when/while they were playing cards. 他们在打牌的时候有人闯进了房子。 2.当表示两个长时行为与情况同时发生时,用while而不用when. i.e.While you were reading the paper, I was working.你在读报时,我在工作。 3.when引导的从句表示在主句中描述的大背景下发生了另一行为。这时不能用while. i.e.I was asleep in my chair when Dora rang to say she wasn't coming home.我睡觉
6、的时候,多利给我打电话说她不会回家。 4.当表示年龄或是生命中的一段时间时,用when而不用while. i.e.When I was a child we lived in London. 我小时候住在伦敦