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。
  
when常与以下结构连用：
1。be doing+ when表示“正在做某事。。。突然。。。”
Iwas doing my homework when the light went out。
我正在写作业突然停电了。
2。be about to do sth+ when表示“正要做某事。
  。。突然。。。”
I was avout to go out when the telephone rang。
我正外出电话就响了。
3。had done +when表示“刚做完某事。。。突然。。。”
I had just entered the house when it began to rain。
  
我刚进门就下雨了。

  时间状语从句: 
连接时间状语从句的连接词有: 
(1) 连词：when, while, as。 它们都表示当…时候的意思。如果主句和从句的动作同时发生且都是进行时 
态，只用while, as。 其他情况下when, while, as通用。
   
1 When he came in, I was listening to the radio． 
2 As I was wandering in the street, I met her。 
3 While (as) I was watching TV, my mum was cooking in the kitchen。
   
(2) 连词before, after, since, until(till)。 当他们表示时间时，他们既是介词又是连词。 
{ He usually drinks a cup of tea before he goes to school。
   
连词 
He usually drinks a cup of tea before going to school。 
介词 
{ 
He usually drinks a cup of coffee after he comes back from school。
   
连词 
He usually drinks a cup of coffee after coming back from school。 
介词 
{ 
I have lived in Dalian since my son was born in 1999。
   
连词 
通常情况下，主句用现在完成时，从句用一般过去时。 
I have lived in Dalian since 1999。 
介词 
{ 
I will wait here until(till) you finish your job。
   
连词 
I will wait here until(till) 8pm。 
介词 
(3) 词组：as soon as 
{ I will tell him as soon as he comes back。 
As soon as she finishes her homework, she will play tennis。
   
通常情况下，主句用一般将来时，as soon as 引导的从句用一般现在时。