when引导的主句从句的时态

由when引导的时间状语从句，主句用过去进行时，从句用一般过去时。

while通常用在事情同时发生的情况，主句和从句都用过去进行时。

I didn\'t use to like him much when we were at school.

以前我们同学时，我并不太喜欢他。

When I left they were all still hard at it.

我离开的时候他们都还在努力工作。

When is the best time to ring New York?

什么时间给纽约打电话最好？

It was a difficult situation, when you think about it.

仔细想来，当时处境是很困难的。

When they set out they were well prepared.

他们出发时有很充分的准备。