三、which和that引导从句时的用法区别

两者都可指物，常可互换。其区别主要在于：

1、引导非限制性定语从句时，通常要用which。（对主句的补充说明，可去掉）

He has to work on Sundays, which he doesn’t like. 

他得在星期天工作，他是不喜欢这样的。

2、直接放在介词后作宾语时，通常要用which。

She may be late, in which case we ought to wait for her. 

她可能晚到，那样我们就要等等她。

注：有时“介词+which”引导的定语从句可换成“介词+which+不定式”结构：

He had only the long nights in which he could study. 

=He had only the long nights in which to study. 

他只有漫漫长夜可用来学习。