when引导的时间状语从句的主句可以使用过去进行时,过去完成时.如：
I was leaving when the telephone rang.
The bus had already gone when I got to the bus station.
when引导的时间状语的从句可以使用一般过去时,一般现在时.
I will call you when I go to Shanghai.
I was leaving when the telephone rang.