在定语从句中，which和that在指代事物时，一般可以互换使用。在非限制性定语从句中则不能用that，要用which。which引导的定语从句可以修饰前面的整个主句。
例句说明
1、A shop should keep a stock of those goods which sell best.
商店应存有最畅销的货物。
2、This is the family which is planning to move to the city
这是要搬进城里的一家。
3、This is a reason which we must not forget.
这是我们不能忘记的一个原因。
4、When deep in thought, which he often was, he would forget all around him.
他常常陷入沉思，这时他就会忘掉周围的一切。
5、He bought a book, which was written by LuXun, and which he decided to give to his friend.
他买了本鲁迅写的书，他决定送给朋友。
6、There is a special parasite which gives rise to the itch.
有一种特别的寄生虫会导致皮炎。
7、He told a story which moved us deeply.
他讲了一个使我们深受感动的故事。
8、In the presence of so many people he was little tense, which was understandable.
在那么多人面前他有点紧张，这是可以理解的。
