时间状语从句是表示时间关系的从句。

 when 表示点时间时，从句中用短暂性动词；表示段时间时，用持续性动词。

  我到家的时候，全家已在吃晚饭。（ when 表示点时间）  When I got home，my family were already having dinner ,

  当他们还在说笑的时候，老师进来了。（ when 表示段时间）  When they were still talking and laughing，the teacher came in ,

  当他看见她的时候，就挥手打了个招呼。（ when 表示点时间）  He waved a hello when he saw her ,

  当你认为自己一无所知的时候，就开始知道一些事情了。（ when 表示段时间）  When you think you know nothing，you begin to know something ,

当 when 意思是正当时候（ and at that moment ）时， when 只能跟在前一分句之后。

  他正要上床，忽然门铃响了。  He was about to go to bed when the door bell rang ,

  他们正看着世界杯比赛，突然灯灭了。  They were watching the World Cup when suddenly the lights went out ,

  他们刚到家，天就开始下雨了。  They had just arrived home when it began to rain ,

有时 when 表示虽然，尽管的含义，相当于 although 或者 since 。

  尽管可以打的，但他还是步行。  He walks when he might take a taxi ,

既然你不宽恕别人，你自己又怎能希望得到别人的宽恕呢？  How can you hope for mercy yourself when you show none?,

while 通常表示一段时间，从句中宜用持续性动词作谓语。

  趁热打铁。  Strike while the iron is hot ,

  她在看报的时候睡着了。  She fell asleep while she was reading the newspaper ,

while 有时可以作并列连词，表示对比，可译成”而”。

  我喜欢英语而他却喜欢数学。  I am fond of English while he likes maths ,

  我们睡觉而上尉担任警戒。  We slept while the captain kept watch ,

while 有时可引导让步状语从句，意思是虽然。

  虽然他们都爱他们的孩子，但却对他们要求严格。  While they love their children，they are strict with them ,

虽然 during 与 while 意思很相近，但是 during 是介词，不能引导从句。 

演出期间请不要站起来。Please remain seated during the performance,

他在夜间被送到医院。He was taken to the hospital during the night,

as 表示点时间时，从句中用短暂性动词；表示一段时间时，用持续性动词。 as 和 when 两者经常可以通用。

小偷在超市行窃时被逮住了。The thief was caught as he was stealing in the supermarket ,

The thief was caught when he was stealing in the supermarket ,

吉姆离开会议室时候我看到了他。I saw Jim as he left the meeting room ,

I saw Jim when he left the meeting room ,

as 表示一边一边，强调从句和主句中两个动作交替进行或同步进行。

他们边走边聊。They talked as they walked ,

他一边走，一边不时地往后看。He looked behind from time to time as he went ,

 as 表示随着

随着时间的推移，天气变得越来越暖了。as time goes on，it's getting warmer and warmer ,

随着春回大地，百花开始绽放。as spring warms the earth，all flowers begin to bloom ,

when，或者while，或者as 只有当从句表示的是一段时间，即其谓语动词有持续性特征时，这三者可以通用互换。

我在吃早饭的时候从收音机里获悉这一消息。I got the news on the radio when I was having breakfast ,

I got the news on the radio while I was having breakfast ,

I got the news on the radio as I was having breakfast ,

as 更强调同一时间或紧接的一前一后或伴随着的变化。

我们听着歌星边弹吉它边演唱。We listened to the singer sing as he played the guitar ,

你一张嘴我就知道你要说什么。I thought of it just as you opened your mouth ,

随着他年龄的增长，他变得越来越不活泼了。as he grew older，he became less and less active ,

状语从句可放在主句之前或之后，放在主句之前时一般用逗号与主句分开。有时可置于主句中间，前后用逗号。

when 更强调特定时间，还可表示从句中的动作先于或后于主句的动作。

当我把多余的票给了玛丽时，我才发现我自己的票已不知去向。（从句动作发生在主句之前，注意主从句的时态）When I had given Mary the spare ticket，I found my own already gone ,

当我最终赶到那里时，会议已开始十分钟了。（从句动作发生在主句之后，注意时态）When I finally got there，the meeting had been on for ten minutes ,

while 从句只能表示延续的动作或状态或主从句中动作的对比。

她以为我在谈论她女儿，而事实上，我在谈论我的女儿。She thought I was talking about her daughter，while in fact，I was talking about my daughter ,

若表示两个短促动作几乎同时发生时，用 as 场合多于 when 。

他结束讲话时，观众爆发出雷鸣般的掌声。as he finished his speech，the audience burst into applause ,

whenever 是 when 的强势语，它描述的不是一次性动作，而是经常发生的习惯性动作。

无论你何时来都欢迎。You are always welcome whenever you come ,

每当我们遇上困难的时候，他们就来帮我们。Whenever we met with difficulties，they came to help us ,