when 能引导什么从句

　　例句：

　　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。

　　She was writing a letter when I came in.

　　当我进来时，她在写信。

　　The students got up when the bell rang.

　　铃声一响，学生们就起床了。

　　I’ll ring you up when I arrive in Beijing.

　　我一到北京就给你打电话。