715　when可以表示and then和at that time/moment

◎ when有时可表示and then或at that time/moment，意为“这时突然，就在那时”，引导的从句要放在主句后。

He was just going out when the telephone rang. 他刚要出去，电话就响了。（=and just then）

She was walking down the road when she heard someone singing. 她正在路上走着，突然听见有人在唱歌。（=and at that time）

He was reading a book when he saw a bird flying past the window. 他正在读书，这时看见一只鸟从窗前飞过。（=and at that moment）

He left the army in 2001，when he went back to his hometown. 他是2001年退伍的，而后回了家乡。