首先，that和which在从句里都可以做宾语和主语，做宾语时可以省略。在定语从句中,有种说法叫"关宾省",意思是关系代词在限制性定语从句中作宾语可以省略,不管是that，which，还是whom。但是which能引导非限制性定语从句，that则不行。

　　其次，记得以下只能用that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。

　　最后，请注意以下几种只能使用which的情况:

　　1. 在“介词＋关系代词”结构中，关系代词必须用which。

　　如： The house in which we live is very large. 我们住的房子非常大。

　　This is the reference book of which the teacher is speaking. 这就是老师正在谈及的那本参考书。

　　注意： 如果介词不放在修饰事物的限定性定语从句的句首， which就可换为that， 例如： This is the question which/that we've had so much discussion about. =This is the question about which we've had so much discussion. 这就是我们已经多次讨论过的问题。

　　2. 先行词为“those＋表事物的复数名词”时， 关系代词通常只用which而不用that。

　　如：you should grasp well those skills which may be used in the future work.

　　最后，记得这个特殊情况吧：当先行词是表示原因或方式的名词时，可以用that引导定语从句，相当于介词+which，that可以省略。

　　如： She likes the child for the very reason that (=for which) she loves his father. 她喜欢那个孩子因为她爱着孩子的父亲。

　　He didn't like the way that (=in which) she speaks to his mother. 他不喜欢她对他的母亲那样说话。

　　Imagine the speed that (=at which) he drives the car. 很难想象，他开车开得那么快。

　　The reason (that /for which /why) he came here was to ask for our help.他来这里的原因是寻求我们的帮助。