引导定语从句的关系词有
　　1． 关系代词 who, whom（代人）；which（代物）；that, whose, as （代人或物）。它们的数和人称要和先行词一致，格取决于它们在句中充当的成分，如：
　　　　　A machine is a kind of device which (that) helps to do work.
　　　　　Those who want tickets should go to the office.
　　　　　The technician whom we met yesterday had worked out a new automatic device.

　　先行词前有限定词 all, any, every, (a)few, no, only, some, very 或序数词或形容词最高级等修饰时，或先行词是指物的不定代词 all, little, few,much, none 或 -thing 构成的复合不定代词时，一般只用关系代词 that,如：
　　　　Any boy that wants to succeed must work hard.
　　　　There is not much that can be done.
　　　　I've made up my mind, and nothing (that) you say will change it.

　　先行词是指示代词 such 或 same 时，只用 as, 先行词被指示形容词 such 或 same 修饰时，通常用as,如：
　　　　This book is not such as I expected.
　　　　She said the same as she said before.
　　　　I've never seen such dogs as you describe.
　　　　I'll wear the same dress as I did last time.

　　2． 关系副词有：when, where, why, 它们都在从句中作状语，其先行词总是表示时间、地点或理由的名词，如：
　　　　The days when they had to import grain from abroad have now passed.
　　　　Air moves from places where the pressure is higher to places where the pressure is lower.
　　　　He refused to tell the reason why he did it.

　　限制性和非限制性定语从句 (Restrictive and Non-restrictive Clause)
　　限制性定语从句是主句中的先行词不可缺少的一部分，如果省去，主句的意义就不完整或不明确，因此，它与主句关系十分密切，不用逗号与主句分开；非限制性定语从句只是对主句中的先行词作附加说明，如果省去，主句仍能表达明确、完整的概念，因此，它与主句间有一逗号分开，如：
　　　　Don't you know the people who live next door?
　　　　The problem, which is complicated, has been solved.

　　That 不用于引导非限制性定语从句；先行词是主句或主句的一部分时,用 which 或 as 引导非限制性定语从句，如从句放在主句之前则用 as 而不用which,如：
　　　　He changed his mind, which made me very angry.
　　　　He was a foreigner, as / which I knew from his accent.
　　　　As was expected, he performed the task with success.

　　关于 “Noun / Pronoun + Preposition + Which / Whom / Whose” 结构
　　　　The resistance of a conductor depends on the kind of substance of which it is made.
　　　　Isn't she the girl with whom you visited the exhibition the other day?

　　关系代词 whom 和 which 常常作介词 of 的宾语，用“名词／代词／数词＋of＋whom / which”结构，如：
　　　　He is reading a book, the name of which I don't know.
　　　　Her parents, both of whom are teachers, are very strict with her.
　　　　In our class there are forty students, four of whom are from Africa.

　　Whose 指物时常可用 of which 代替，这时 whose 所修饰的名词放在 of which 之前，如：
　　　　We had a meeting whose purpose was completely unclear.
　　　　= We had a meeting the purpose of which was completely unclear.