as 强调主、从句动作同时发生 表示"一边......一边......"之意 　　He was nine when his father died.

　　他父亲去世时他才九岁。

　　When the meal was finished，Rachel washed up and made coffee.

　　吃完饭，雷切尔洗了餐具并泡了咖啡。

　　They arrived while we were having dinner.

　　他们到达时我们正在吃晚饭。

　　While I played the piano，my sister did her homework.

　　我弹钢琴的时候姐姐在做作业。

　　As time passed，things seemed to get worse.

　　随着时间的推移，情况似乎变得更加糟糕了。

　　I saw my mother as I was getting off the bus.

　　我下公共汽车的时候看见了我妈妈。

　　[名师点津]　when在be about to do...when...，be doing...when...，had done...when...，be on one's way...when...，be on the point of doing...when...等结构中，作"正在那时"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.

　　他刚要走就有人敲门。

　　2．till和until

　　(1)主句谓语动词为延续性动词时，主句和从句都用肯定句式，意为"一直到"，表示主句动作一直延续到until/till从句表示的时间才终止；主句谓语动词为短暂性动词时，主句用否定式，从句用肯定式，意思是"直到......才......"，表示主句动作到until/till从句表示的时间才开始。

　　He studied until/till it was 9：00.

　　他一直学到9点。

　　He didn't study until/till it was 9：00.

　　他到9点才学习。

　　(2)until和till意义相同，通常可以换用，但在强调结构、倒装句或句首时，一般用until。

　　She didn't go to bed until/till her mother returned.

　　＝It was not until her mother returned that she went to bed.

　　＝Not until her mother returned did she go to bed.

她直到妈妈回来才上床睡觉。