when引导什么从句
一、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。
二、引导条件状语从句
when引导条件状语从句相当于if/in case引导的条件状语从句。
（1）When there is no gravity，our feet can no longer stay on the ground.如果没有重力，我们的脚就不能再地面上站稳。
（2）Turn off the switch when anything goes wrong with the machine.如果机器发生故障，就把电闸关上。
三、引导原因状语从句
when 引导原因状语从句时，相当于since/now that引导的原因状语从句。
（1）How can they learn anything when they spend all their spare time watching television?他们把所有的空闲时间都用来看电视了，还能学到什么东西呢？
（2）Why do you want a new job when you have got such a good one.既然你已经有了这么好的一份工作，为什么还要再找新工作呢？
四、引导让步状语从句
when引导的让步状语从句相当于though或although引导的让步状语从句。
（1）He usually walks when he might ride.虽然有车可乘，但他通常是步行。
（2）The little girl can tell right from wrong when she is only twelve.这个小女孩虽然只有十二岁却能分清是非。
五、引导定语从句
when引导定语从句时，有时可用in which或on which来替代。
（1）We’ll put off the picnic until next week,when the weather may be better.我们要把野餐推迟到下周，那时可能天气好些。
（2）I’ll forget the day when I joined the Party.我将永远不会忘记我入党的那一天。
六、引导名词性从句
1.引导宾语从句
Please tell us when his father will return from abroad.请告诉我们他父亲什么时候从国外回来。
2.引导主语从句
When they will leave for Australia hasn’t been decided.他们什么时候动身去澳大利亚还没有决定。
3.引导表语从句
The question is when they will get so much money to set up the factory.问题是他们将在什么时候弄到这么多钱来把这个工厂办起来。