when,while,as都能引導多種狀語從句：
一、when常見考點
1、“當……時候”,引導時間狀語從句。
When he was a young man，he was fond of hunting. It was already 10：00PM，when I finished the work.
2、“正在那時，這時”等於and at that time，引導時間狀語從句。
We were having a meeting when someone broke in. We were about to leave when it began to rain.
3、“既然”等於since 或considering that， 引導原因狀語從句。
It was foolish of you to take a taxi，when you should easily walk there in five minutes. How can they learn anything when they spend all their spare time watching TV?
4、“雖然，然而，可是”引導讓步狀語從句。
He usually walks when he might ride. They had only three chairs when they need five.
二、while常見考點
1、“當……時候”， 引導時間狀語從句(從句動作必須是延續性的)。
Please don't talk so loud while others are working. While I was cleaning the window，my finger was seriously cut.
2、“雖然，儘管”相當於although，引導讓步狀語從句(常把while放在句首)。
While she is a top student， she has some shortcomings. While the Internet is bridging the distance between people， it may also be breaking some home or will cause other family problems.
3、“只要，如果”等於as long as，引導條件狀語從句。
There is hope while there is life. While you study hard， your effort will pay off.
4、“而，然而”，表對比或相反的情況。
He likes pop music，while I am fond of folk music. Kids talk frequently about what they can do，while many adults do the opposite， tending to talk about what they can't .
三、as常見考點
1、“一邊……一邊”或“隨著……”，引導時間狀語從句(as引導的`從句內部的動作與主句內動作同步)。
John sings as he works. (一邊……一邊)
Jame‘s blindness was getting worse as he was getting older.(隨著……)
2、“因為”， 引導原因狀語從句。
As you weren‘t there， I left a message. Perhapshe will need some help as he was ill.
3、“儘管，即使，雖然” 引導讓步狀語從句(用於形容詞，副詞或名詞後構成部分倒裝)。
Young as I am， I already know what career I want to follow. Much as I like you， I couldn't live with you. Child as Tom is ， he has been a success.
