When they heard the news, they were very surprised.

　　当他们听到那个消息时，他们非常惊讶。

　　When he had finished his homework, he took a short rest.

　　完成作业后，他休息了一会儿。

　　[名师点津]　

　　when可用作并列连词，意为"这时，那时"，相当于and at this/that time。常用于下列句型：①was/were doing sth....when...正在做......这时......

　　②be about to do sth./on the point of doing...when...正要做......这时......

　　I was watching TV when someone knocked at the door.我正在看电视，这时突然有人敲门。

　　I was about to go to bed when he called me up.

　　我正要上床睡觉的时候，他打电话来了。

　　2. while引导时间状语从句时，意为"当......的时候"，从句的谓语动词必须是延续性动词。强调主句和从句的动作同时发生或相互对应。　

　　Don't talk so loud while others are working.

　　其他人工作时，请勿大声说话。

　　While the child was playing with his toys, his parents were watching TV.

　　当那个孩子玩玩具时，他父母在看电视。

　　3．as引导时间状语从句时，常表示两个动作同时发生，常译作"一边......一边......；随着"。

　　The students took notes as they listened.

　　学生们边听课边记笔记。

　　As you grow older, you will know better and better about yourself.

　　随着年龄的增长，你会越来越了解你自己。

　　二、过去完成时

　　1．构成：had＋过去分词

　　2．用法

　　(1) 表示在过去某个时间或动作之前"过去的过去"，即通常简称为就已经发生的动作。句中常有by，by the end of等介词(短语)和by the time，before，when，until等引导的状语从句。

The concert had already begun when I got there.