英语中,连词when表示"当……时候",引导的时间状语从句表示与主句动作同时或不同时,从句动词用延续或非延续性都可以.如： I'll telephone you when I get there tomorrow.明天我到那里的时候就给你打电话. 但是在高一新教材Unit 15 "The necklace"中,就出现了when用作并列连词,表示"就在这时"(=and this time/just at that time)的用法.如： Tom was sitting near the fire when he heard a knock at the door.汤姆正坐在火炉边,这时他听到一阵敲门声. I was just going to speak when the doorbell rang.我正准备讲话,这时门铃响了. 当并列连词when用于"就在这时,正在这时"的用法时,它所引导的并列句表示动作发生后,另一个并列句所表示的动作之后,一般不可以用其他词代替,常用于以下结构： 1.was/were doing +when意思为"就在这时,正在这时".如： I was walking in the street when someone called me from behind.我正在街上走,就在这时有人从后面叫我. They were discussing the problem when the teacher came in.他们正在讨论问题,就在这时老师进来了. 2.was/were about to do...when意思为"正要/正准备做某事,就在这时……".如： She was about to go out when an unexpected visitor came.她正准备出去,这时来了一位不速之客. We were about to set out when it began to rain.我们正要动身,就在这时开始下雨了
3.be+表示状态的介词短语+when意思为"正在做某事,就在这时……".如： They were at work in the office that afternoon when a big fire broke out.那天下午他们正在办公室工作,就在这时一场大火发生了. We were on duty in the classroom when a boy rushed in.我们正在教室里值日,就在这时一名男孩冲了进来. Li Ping was on his way home when two boys stopped him.李平正往家走,突然两个男孩拦住了他.
注意：在上述三种用法的结构中,when引导的句子不能放在句首.
