when除了用来引导主语从句、宾语从句、时间状语从句、表语从句和定语从句
常用于以下句型中：(1)、主语 + be doing… when…意思是“正在做某事这时……”(2)、主语 + be about to do…when…(3)、主语+be on the point of (doing) …when…意思是“正要去做某事这时……”
例句：One day Chuck was on a flight across the Pacific Ocean when suddenly his plane crashed. 有一天，Chuck正在太平洋上飞行，这时他的飞机突然爆炸了。
I don’t know when he will arrive. 我不晓得他什么时候到达。
I thought of the happy days when I was in Beijing. 我想起了在北京时的那段快乐日子。
I believe the time will soon come when there will be no weapons in the world. 我相信世界上没有武器的日子不久就会到来。
I was walking along the river when I heard a drowning boy cry for help. 我正在河边行走，这时我突然听到一个落水男孩求救。
I was about to leave when it began to rain. 我刚要离开，这时下雨了。
I had just finished my exam paper when the bell rang， announcing the class was over. 我刚做完作业，下课铃声响了
连接副词when， where， why， how引导的主语从句。这些连接副词相当于副词，在从句中充当状语。
Why he did this is not known.
When he will come is still unknown.
It is not known why he did this.
How he succeeded is unknown to us.
Where we should hold the meeting needs to be discussed.
