引导时间状语从句
1.表示“当……的时候”，相当于at the moment when。
（1）When we got to the cinema,the film had been on for half an hour.当我们到电影院时，电影已经放映半小时了。
（2）She was writing a letter when I came in.当我进来时，她在写信。
2.表示“一……就……”，相当于as soon as。
（1）The students got up when the bell rang.铃声一响，学生们就起床了。
（2）I’ll ring you up when I arrive in Beijing.我一到北京就给你打电话。
3.表示“就在这时；当时”，相当于just at the moment或just then。
（1）We were about to start out when it began to rain heavily.我们正要启程，就在这，天下起了大雨。
（2）He had just returned from one business trip when he was asked to make another one.他刚刚出差回来，这时，又叫他再次出差。
4.表示“每当；每次”，相当于every time或whenever。
（1）She always turns to us for help when she is in trouble.每当她遇到困难，她总是向我们求助。
（2）It is freezing cold here when it snows.每当这儿下雪，天气就十分寒冷。
5.表示“当……之后”，相当于after the time that。
（1）We went home when the film was over.电影结束以后，我们回家去了。
（2）When she got home，she started to prepare supper.她回到家后，开始准备晚饭。