第1个回答 2011-05-04
when-----(at or during the time that )既可以表示在某一点的时候，又可表示在某一段时间内，主句与从句的动作或事情可以同时发生也可以先后发生。如果表示从句的动作在主句的动作“之前”或“之后”发生时，多用when（=after）, 不用as或while。此外，when还含有“at that moment”的意思（= and then），引起的句子不能放在句首，也不能用as, while来替换。
It was raining when we arrived.(指时间点)
When we were at school, we went to the library every day.(在一段时间内)
We were watching TV when suddenly the lights went out.我们正在看电视，这时灯突然灭了。
They had just arrived home when it began to rain.他们刚到家，这时天就开始下雨了。
while------while意思是“当……的时候”或“在某一段时间里”。主句中的动作或事情在从句中的动作或事情的进展过程中发生，从句中的动词一般要用延续性动词。在when表示a period of time时，两者可以互换。
当主从句之间表示转折或对比关系时，多用while, 不用as或when
Please don’t talk so loud while others are working.
He fell asleep while/when reading. Strike while the iron is hot.(用as或when不可，这里的while意思是“趁……”)
She thought I was talking about her daughter, while in fact, I was talking about my daughter.她以为我在谈论她女儿，而事实上，我在谈论我的女儿。
1)when引导从句时，主从句的动作有先有后，也可以同时进行,从句的动作可以是持续性的，也可以是短暂的。如：
When I got to the airport, the plane had already taken off. （主先从后）（短暂性）
When I lived there, I used to go to the seaside on Sundays. (同时) （持续性）
When the movie ended, the people went back. （从先主后）
2)while侧重主从句动作的对比，且从句的动词必须是持续性的。如：
While we were chatting she was looking at the time table on the wall．
3)as引导从句时侧重主从句动作同时或几乎同时进行，从句的动作可以是持续性的，也可以是短暂的。如：
Sometimes I watch TV as I am having breakfast．
4）when和while还可以是并列连词，意思分别是“就在这时”，“然而”。如：
I was having a rest on the sofa when the telephone rang．
They were surprised that a child should work out the problem while they couldn’t .注：
并列连词when常用与以下句型中：
①…was/were doing…when…（正在做…突然）
②…was/were about to do…when…（刚要做…突然）
③…was/were on the point of doing…when…(刚要做…突然)
④…had just done…when….(刚一…就)
⑤Hardly/Scarcely had done…when…(刚一…就)
(2)before引导从句时,词义非常灵活，注意下列句子中的before的词义：
1.Before I could get in a word, the tailor had measured me.(还没来得及…就)
2.We hadn’t run a mile before he felt tired.(还没…就)
3.We had sailed 4 days before we saw land.(…才…)
4.Please write it down before you forget it.(趁还…没就)