1、由when引导的时间状语从句。举例如下：You were about to leave when she came in.你就要离开，就在那时她进来了。
2、主句动作发生时，when从句动作正进行。举例如下：When climbing up the hill, we snatched at a rock.在向山上爬的过程中，他一把抓住了一块岩石。
3、when从句动作发生时，主句动作将会发生。举例如下：I will tell her about it when I see her.我见到她时就告诉她这件事。
