【问】 We were about to start when it rained.(章振邦主编《新编英语语法》上册，P. 200)请问when it rained用得是否正确?

【答】 我们在when³条中已经谈到这种句型中的when=at that moment，并表示when分句里的动作的突然性。因此，when分句里的谓语动词必须是瞬间动词，而不可能是持续性动词，如rain。请看下面的实例：

1. He was on the point of leaving when someone knocked at the gate.

他刚要走，有人敲门。

2. The government________ to approve the use of wide-spread surveillance when the JusticeDepartment took objections.

A. is going B. had been

C. was about D. is coming(TOEFL，76.2)

3. She had just finished dressing when her guests arrived.

她刚穿戴完毕，客人们就来了。

4. Johnsy was sleeping when they went in.(O.Henry)

约翰西正在睡觉，他们突然走了进来。

5. I was wandering through the streets when I caught sight of a tailor's shop. (M. Twain)

我正沿着街道漫步，突然看见了一个裁缝店。

6. The planes had been planting seed for nearly a month when it began to rain. (NCE 2，L.62)

飞机播种了将近一个月才开始下雨。

因此，《新编英语语法》例句中的when it rained应改为when it began to rain.