当定语从句所修饰的先行词为物时，关系代词可用which或that。例如：
The pen that/which I am using is quite good. 我正在使用的那支钢笔相当好。
定语从句中只能用that，不能用which的几种情况：
1. 当先行词为all, much, little, none及由any, every, some, no 所构成的复合不定代词时。
如：We should do all that is useful to the people. 我们应该做一切有益于人民的事情。
Please tell me anything that you know about the matter. 有关此事，凡是你所知道的请告诉我。
2. 当先行词被any, few, little, no, all, much, some 等词修饰时。
如：You can take any seat that is free. 任何空着的座位你都可以坐。
There is little work that is fit for you. 几乎没有适合你的工作。
3. 当先行词为序数词或被序数词所修饰时。
如：When people talk about the cities of China, the first that comes to my mind is Beijing. 人们谈论起中国的城市的时候，我首先想到的是北京。
This is the fourth film that has been shown in our school this term. 这是我们校本学期放映的第四部电影。
4. 当先行词为形容词最高级或被形容词最高级修饰时。
如：The best that I could do was to apologize. 我唯一能做的就是赔礼道歉了。
This is the most interesting story book that I have ever read. 这是我读过的一本最有趣的故事书。
5. 当先行词被the very, the only, the last, the just, the same等修饰时。
如：This is the very book that I’m looking for. 这正是我在找的书。
The only thing that we could do was to wait. 我们唯一能做的事就是等待。
注意：先行词被the same修饰时，关系代词也可用as。
如：I need the same book that / as you have. 我需要有你一样的书。
6. 当先行词既包含人又包含物时。
如：They are talking of the heroines and their deeds that interest them. 他们正在谈论他们感兴趣的女英雄及她们的事迹。
7. 当先行词是疑问代词who，which，what或主句以这些词开头时。
如：Who that has ever worked together with him doesn’t admire him? 曾经和他一起工作过的谁不钦佩他？
Which is the star that is nearer to the earth? 哪个是离地球比较近的星星？
8. 当先行词为主句表语或关系代词为从句表语时。
如：That’s a good book that will help you a lot. 那是本对你很有帮助的书。
Our school is no longer the place that it used to be. 我们的学校现在已不是过去的那个样子了。
9. 先行词为time时，当time表示次数，引导词用that, 可省略。当表示时间，可用that或when引导，都可省略。
如：I do remember the first time (that) I had ever heard the sweetest voice in the world.我很清楚地记得当我第一次听到世界上最美的声音的时候。
I did't remember the exact time（when/that）I arrived in Shanghai last month. 我不记得上个月到达上海的确切时间了。
