【问】 您在前面讲过，when分句有时并不是充当时间状语从句来修饰前面的分句，倒是前面的分句为when分句提供了时间背景。请问在什么条件下when分句具有此种用法?
 【答】 当位于句末的when分句具有此种用法时，它前面的分句大致有三种情况：
 A. when分句前面的分句使用过去进行时：
 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. I was thinking of this when I heard my name called.(许国璋《英语》，第二册，P.176)
 我正想着这件事，突然听到叫我的名字。
 3. He was going to throw the dish away when he noticed that the penicillium colony wassurrounded by an area completely clear of the pathogenic bacteria. (CBSE)他正想把这个碟子扔掉，这时他突然发现青霉素的周围完全没有病原菌了。
 4....I happened to be walking in my garden when I noticed a huge number of snails takinga stroll on some of my prize plants. (NCE 3，L.23)
 我碰巧在庭园里散步，突然看到许许多多的蜗牛正在一些我所喜欢的植物上爬动。
 5. “I was lying on my back reading when I saw the snake. I knew that moving was out of____.”
 A. the bargain B. the question
 C. the chancel D. the risk( Nelson，450A)
 6. She was thinking that she must reprimand her daily maid the next morning for such amonstrous piece of negligence，when she remembered that she had gone shopping afterthe maid had left and she knew that she had turned both keys in their locks. (NCE 3，L.58)
 她正想着第二天早上一定要把打日工的女仆好好训斥一番，责备她竟然做出如此粗心大意的事来，这时她突然想起她是在女仆离开之后才出去购物的，而且她把两把
 锁都锁上了。
 B.when分句之前的分句用了was(were) about to，was (were) on the point：
 1. We were about to start when it began to rain.
 我们刚要出发，就开始下雨了。
 2. He was on the point of leaving when someone knocked at the gate.
 他刚要走，有人敲门。
 3.见when⁵例2。
 C.when分句之前的分句用了过去完成时：
 1.We had just fallen asleep when the bell rang.
 我们刚刚入睡，突然铃声响了起来。
 2. I had nearly reached the town，when the young man suddenly said，very slowly，‘Doyou speak English?’(New Concept English，Book 2，L.14)
 在快到达城镇时，那个年轻人突然慢慢地说：“你会说英语吗?”
 3. The planes had been planting seed for nearly a month when it began to rain. (New Con-cept English，Book 2，L. 62)
 飞机播种了将近一个月才开始下雨。