①若主句谓语动词是短暂动作，从句时间状语指一段时间，三者可通用。

　　[举例]While/As/When I was walking in the street, I saw him.

　　当我在路上散步时，我看见了他。

　　②as 意为"边......边......"或"与......同时"，着重表示两动作同时发生；when意为"当......时刻或时期"，它即可以指时间点，也可以指时间段，从句的谓语动词可以是非延续性的，也可以是延续性的；while仅指时间段，从句的谓语动词只能是延续性的。

　　[举例]They talked as he walked

　　 他们边走边谈。

　　When I opened the old book, I found a picture of him.

　　当我打开这本旧书时，我发现了他的一张照片。

　　Somebody broke in while we were out.

　　在我们出门不在的时候，有人破门而入。

　　[经典考题]Don't be afraid of asking for help_____ it is needed.(NMET2003)

　　A. unless B. since C. although D. when

　　[点拨]答案D 根据句意可确定应选when。句意为"当需要时不要害怕求助"。

　　7.My mother has always made sure we eat very healthily，and fresh fruit and vegetables are a very important part of our diet.

　　我妈妈总是保证让我们吃健康食物，新鲜的水果蔬菜是我们日常食物的重要组成部分。

　　（1）make sure 弄明白，务必确保，后可接宾语从句或接of或about。

　　[举例]I think I locked the door but I'll just go back and make sure

　　 我认为我锁上门了，但得回去核实核实。

　　Please make sure the house is locked.

　　务必确保房门已锁上。

　　Have you made sure of the time of the train?

　　你搞清楚火车的时间了吗？

　　[辨析]make sure 与be sure

　　make sure 意思是"（把某事）搞清楚；确定；查明"，后接that引导的宾语从句和

　　介词of/about。但make sure 后不跟不定式或if/whether 引导的从句；be sure意思是"确信的；有把握的"，后接介词of/about, that-clause, what-clause和疑问词+不定式。

[举例]I'm sure of his success=I'm sure that he will succeed.