hen,while,as引导时间状语从句的区别
when,while,as显然都可以引导时间状语从句,但用法区别非常大.
一、when可以和延续性动词连用,也可以和短暂性动词连用；而while和as只能和延续性动词连用.
① Why do you want a new job when youve got such a good one already?（get为短暂性动词）你已经找到如此好的工作,为何还想再找新的?
②Sorry,I was out when you called me．（call为短暂性动词）对不起,你打电话时我刚好外出了.
③Strike while the iron is hot．（is为延续性动词,表示一种持续的状态）趁热打铁.
④ The students took notes as they listened．（listen为延续性动词）学生们边听课边做笔记.
二、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都可使用.
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 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