when引導的從句
　　when除了用來引導主語從句、賓語從句、時間狀語從句、表語從句和定語從句，還有一種用法值得關注，那就是when引導并列分句，意思是“這時突然;就在那時”，強調另一個動作的突然發生。常用于以下句型中：(1)、主語 + be doing… when…意思是“正在做某事這時……”;(2)、主語 + be about to do…when…;(3)、主語+be on the point of (doing) …when…意思是“正要去做某事這時……”
　　[例句]
　　One day Chuck was on a flight across the Pacific Ocean when suddenly his plane crashed. 有一天，Chuck正在太平洋上飛行，這時他的飛機突然爆炸了。
　　I don’t know when he will arrive. 我不曉得他什么時候到達。
　　I thought of the happy days when I was in Beijing. 我想起了在北京時的那段快樂日子。
　　I believe the time will soon come when there will be no weapons in the world. 我相信世界上沒有武器的日子不久就會到來。
　　I was walking along the river when I heard a drowning boy cry for help. 我正在河邊行走，這時我突然聽到一個落水男孩求救。
　　I was about to leave when it began to rain. 我剛要離開，這時下雨了。
　　I had just finished my exam paper when the bell rang, announcing the class was over. 我剛做完作業，下課鈴響了。
　　I thought of the happy days when I was in France. 我想到了在法國時那段快樂日子。
　　I believe the time will soon come when there will be no weapons in the world. 我相信世界上沒有武器的日子不久就會來臨。
　　[比較]
　　I was walking along the street when I caught sight of a tailor’s shop. 我正在街上走，這時我看見一家裁縫店。
　　I was walking along the streets. Just at the moment I caught sight of a tailor’s shop. 我在街上走。就在那時我看見一家裁縫店。