When ① 多数跟短暂性动词，有时也跟延续性动词，主句动词和从句动词几乎是同时发生。当….的时候 Eg: when I opened the window，I saw him come up. When the teacher came in,the light went out. ② 正在或正要做某事的时候，突然发生另外一个动作…. 句式:be doing 或be about to do when+did Eg:I was walking along the street when I met him. I was about to fall asleep when my sister came in. 翻译：我正要出门，天下起雨来了。 I was about to go out when it began to rain.