Ⅰ. 引导定语从句的关系代词有：who, whom, whose(一般指人)，which(一般指物)，that ( 指人或物)等。关系副词有：where(地点)when(时间)， why(原因)等。
Ⅱ. that 在从句中指物，也可指人，可作主语和宾语。如：
① Water that is polluted often causes serous illness. 受污染的水常会引起重病。(that 在句中指物，用作主语)
② Do you have everything that you need? (that在从句中指物，用作宾语) 你所需要的东西都有了吗?
③ He is the man that they talked about just now. (that 在从句中指人，用作宾语) 他时刚才他们谈论的那个人。
④ Do you know the man that / who spoke just now? 你认识刚才讲话的人吗?
Ⅲ. who 和whom 在句中指人，分别作主语和宾语。如：
① A doctor is a person who looks after people’s health. ( who 在从句中用作主语) 医生是保护人们健康的人。
② I have just met a lady whom I saw last week. (whom在从句中用作宾语) 我刚遇上一位我上星期见过的人。
Ⅳ.which 在从句中指物，可作主语和宾语。如：
① My aunt was not on the train which arrived just now. (which 在从句中作主语) 我阿姨不在刚才到达的那列火车上。
② This is the coat which you wanted.(which 用作宾语)这就是你要的那件外套。
Ⅴ. whose 在从句中多指人，也可指物，用作定语。如：
① You are the only one whose advice he might listen to. (whose 作advice的的定语) 只有你的话他可能听。
② I’d like a room whose window looks out over the sea. 我想要一个窗户面临大海的房间。
[注意]：在下述情况下，定语从句中关连词只能用 that. :
Ⅰ. 先行词是 all, everything, nothing, something, anything, little, much 等不定代词时，如：
① Only a scientist could understand all that this pursuit meant.只有科学家知道这追求意味着什么。
② I am sure she has something that you can borrow. 我相信她有你能借到的东西。
③ Everything that we saw was of great interest to us. 我们对见到的一切都感兴趣。
④ There is little that I can use. 我能用的东西几乎没有。
⑤ He saw much that was bad. 他见了很多坏东西。
[注]：①先行词是 something 时，关系代词用 that 或 which 都可以。
②先行词是：someone, anyone, everyone, somebody, anybody, nobody,everybody 时，关系代词用 that 或 who 都可以。
Ⅱ. 先行词被 all, every, very no, some, any, little, much 等修饰时，如：
① I have read all the books that you gave me. 我把你给我的书全都看了。
② He is the very man that came here yesterday. 他就是昨天来的人。
Ⅲ. 先行词被序数词、形容词最高级修饰时。如：
① This is the first composition thathe has written in English. 这是第一篇他用英语写的作文。
② The smallest living things that can be seen under a microscope are bateria. 在显微镜下我们能看见的最小的东西是细菌。
③ ¬This is the best novel that I have ever read. 这是我读过的最好的小说。
Ⅳ. 先行词被 the only, the very, the same, the last 等修饰时。
① That white flower is the only one that I really like.白花是我唯一真正喜欢的花。
② This is the very book that I want to find. 这正是我想要的书。
③ The last place that we visited was the chemical works. 我们最后参观的是化工厂。
Ⅴ. 当有两个或两个以上分别表示人和物的先行词时，定语从句只能用that 与主句连接，而不能用who / whom / which引导。如：
① He talked about the teachers and schools that he had visited . 他谈了关于他访问过的老师和学校的情况。
Ⅵ. 当主句是以 who 或which 开头的特殊疑问句时，关连词只能用that . 而不能用 who / whom / which. 如：
① Who is the person that is standing at the gate?站在门口的那人是谁。
② Which of us that knows something about physics does not know this?我们当中哪一个懂物理的人不知道这个?
[注]：在使用一些固定搭配的短语动词时，that 与其它关系词有时可以互用，其介词不能提前，而必须放在动词之后，这时指物用that, which 均可，指人时用who, whom, that 均可.
① This is the key which / that you are looking for.
② He is a man (whom / that ) you can safely depend on. 他是你能依赖的人。
③ The person whom / that / who he looks after is his mother.