when 可以引导很多从句：
1.宾语从句：例如 I don`t know when he left here.我不知道他上面时候离开的.此时when作连接词引导宾语从句.
2.定语从句：例如 He left his hometown in that year when his father died.他在他父亲死的那一年离开了他的家乡.此时when作连词修饰that year,引导定语从句.
3.状语从句：例如 He will go to the doctor`s when it is sunny.当天晴时,他就会去看医生.此时when引导状语,意思是“当...时候”.
4.主语从句：例如When he left here is a question.他上面时候离开这儿的还没有人知道.此时when引导主语从句,意思是“什么时候”