在这种用法中，when 分句里的谓语动词必须是瞬间动词，不能是持续性动词。通常用于以下三种情况：

      （1）在 when 前面的分句里，谓语动词是过去进行时。如：

       I was thinking of this when I heard my name called. 我正想着这件事，突然听到有人叫我的名字。（摘自许国璋《英语》第二册p.176） 

      （2）在 when 前面的分句里有 was /were about to 或者 was /were on the point of，如：

       We were about to start when it began to rain. 我们刚要出发，就开始下雨了。

       He was on the point of leaving when someone knocked at the gate . 他刚要走，有人敲门。

      （3）在 when 前面的分句里，谓语动词是过去完成时或者过去完成进行时。如：

       I had nearly reached the town, when the young man suddenly said, very slowly, "Do you speak English?"（摘自New Concept English, Book 2, Lesson 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？”

       The planes had been planting seed for nearly a month when it began to rain. （同上, Lesson 62）飞机播种了将近一个月，突然开始下雨。