47.当......的时候

　　when+延续性/短暂性动词

　　while+延续性动词（多用进行时）

　　When I was watching TV，he came in.

　　=When he came in，I was watching TV.

　　While I was in Shanghai，I visited him.

　　48.at the age of和when引导的时间状语可以互换

　　I began to play football when I was five years old.