一、作为从属连词，引导状语从句

1、表示时间，意为“当……时；在……的时候；一 …… 就”。

 (1) When you see him, please say hello to him. 见到他时，代我问他好。

 (2) When I reached the station, the train had left. 当我到达火车站时，火车已经开走了。

 (3) We had hardly fallen asleep when the bell rang. 我们刚刚入睡，铃声就响了起来。

2、表示原因，相当于in view of the fact that; considering that 既然；考虑到。

 How can I help them to understand when they won't listen to me？

 既然他们不愿听我的，我又怎么能帮助他们弄明白呢？

3、表示条件，相当于if。

 How can I get a job when I can't even read or write？如果我连读和写都不会，我怎么能找到工作呢？

二、作为并列连词，引导并列句。

3、表示前面的叙述所没有提到过的信息。相当于after which; and just then (implying suddenness) 意为：一…就；正在这时

(1) He was still smiling when the door opened and his wife came in.

     他正笑着的时候，门突然开了，他妻子走了进来。

(2) We were about to start when it began to rain. 我们刚要出发就开始下雨了。

(3) He was on the point of leaving when someone knocked at the door. 他刚要走就有人敲门。

4、表示对比，相当于although; whereas，意为“既然；然而”。

They have only three copies when we need five. 他们只有三本，而我们却需要五本。

练习：

 B  1. He transplanted the little tree to the garden ____ it was the best time for it.

   A. where B. when C. that    D. until    05上海

D  2. Peter was so excited _______he received an invitation from his friend to visit Chongqing.

   A. where       B. that   C. why         D. when    09重庆

A  3. I had hardly got to the office ___ my wife phone me to go back home at once.

   A. when    B. than    C. until    D. after     12全国2

D  4. How can you expect to learn anything ____ you never listen?

   A. in case   B. even if   C. unless   D. when    06山东

D  5. Why do you want a new job _____you've got such a good one already?

   A. that    B. where    C. which    D. when    98全国

A  6. Jasmine was holidaying with her family in a wildlife park _____ she was bitten on the leg by al lion.

   A. when      B. while      C. since       D. once   04上海

D  7. He was about to tell me the secret __________ someone patted him on the shoulder.

   A. as      B. until       C. while     D. when    02上海

A  8. She had just finished her homework _____ her mother asked her to practice playing the piano yesterday.

   A. when         B. while    C. after       D. since    09福建