讲，表示某个动作正在发生或刚刚发生，另一动作同时发生。

　　He was about to leave when someone knocked at the door.

　　他刚要离开就有人敲门。

　　[即时演练1]

　　(1)选词填空：when, while, as

　　①It's much easier to make friends when you have similar interests.

　　②While/When in Vienna he studied music.

　　③You will grow wiser as you grow older.

　　(2)完成句子

　　①I was_about_to_go_to_bed_when the telephone rang.

　　我正要睡觉时电话铃响了。

　　②They were_watching_TV_when_they learned that the plane was out of touch.

　　他们正在看电视，这时得知飞机失去联系。

　　③He had_just_left_when you came.

　　他刚离开，你就来了。

　　④He dropped the glass as_he_stood_up.

　　他站起来时，把杯子摔了。

　　2．until, till的用法

主句形式 主句谓语 意义 肯定句 用延续性动词 意为"直到......为止"，表示主句中的动作到从句动作发生时结束 否定句 用短暂性动词 意为"直到......才......"，表示主句中的动作到从句动作发生时开始 　　

　　He waited until she had finished speaking.

　　他一直等到她讲完。

　　You can't go home until you finish your work.

　　工作完成后你才能回家。

　　[名师点津]　(1)置于句首或在强调句中时常用until，其他情况until和till可通用。

　　Until you told me I had no idea of it.

　　直到你告诉我，我对此事才有所了解。

　　(2)如果将"not until ..."结构放在句首，那么主句要部分倒装。

Not until his bike looked almost new did he stop cleaning it.