一. 定义：定语由一个句子来充当，那么这个句子就叫做定语从句。
eg: 其中划线部分为定语从句。
二. 分类：分为限制性定语从句和非限制性定语从句，还有间隔性定语从句。 (限制性定语从句)
(非限制性定语从句)
( 间隔性定语从句)
注：定语从句和非限制性定语从句的区别
限制性定语从句无逗号隔开、不可缺少、起修饰限制作用;而非限制性定语从句有逗号隔开、可有可无、起补充说明作用。
三. 构成：定语从句有先行词、引导词和从句构成。
先行词是定语从句所修饰限制的名词或代词。
引导词是用来引导定语从句的关系代词或关系副词。
eg: The old town has narrow streets and small houses that are built close to each other. 此句中先行词为：narrow streets and small houses, 引导词为：that， 定语从句为：that are built close to each other
四.基本原则
定语从句中不能出现与先行词在意义上相一致的词 与book指同一物，所以要去掉。)
五.定语从句中常见考点：
考点一.关系代词和关系副词的辨别
1. 关系代词that(人、物)，which(物)、who(m)(人)、whose(某人的，某物的)、as
2. 关系副词when(表时间)、where(表地点)、why(表原因)
注：关系副词可变为“介词+关系代词”结构(即：关系副词=介词+关系代词)。
eg:This is the mountain village where(=in which)I stayed last month.
3. 怎样选择正确的'关系代词或关系副词 方法一：找出先行词和定语从句中动词，看定从中动词与先行词能否构成习惯搭配。能，用关系代词;不能，用关系副词.
Eg.I will never forget the day定从中动词spent 与先行词
the day 构成spent the day ,所以用关系代词that或which或省略)
系代词。如不缺少，定语从句所表达意思相对完整，用关系副词。
eg:① is the factory ____ made cars . (缺少主语，所以用关系代词that 或which，作主语不能省略)
② is the reason _______ he was late for school.(定语从句he was late for school 表
达意思相对完整，所以用关系副词why 或介词+关系代词for which) 注意：当先行词为case、position、point、situation、society、activity等抽象意义的名词时，常用关系副词where引导定语从句.
eg:He got himself into a dangerous situation where he may lose control of the plane .
考点二. 定从中that 与which的区别
1. 关系代词只用that的情况。
(1)当先行词为不定代词(all、much、little、few、some、any、none、one、something、anything、everything等)或先行词被不定形容词(all、many、some、few、little等)修饰时。 that you want to say for yourself?
(2)当先行词既有人又有物时。 that you are talking about.
(3)当先行词含有序数词或最高级时。
eg: ① will go to Beijing.
②(4)当先行词被only、very、next、last等修饰时。
that I’m looking for.
(5)在who或which引导的特殊疑问句中。 that you lost?
(6)当关系代词在定语从句中作表语时。 that it used to be.
2.关系代词只用which的情况。
(1)引导非限制性定语从句时。
eg:He turned to be a very successful man,(2)在“介词+关系代词”结构中，关系代词只用which。
eg:This is the question they have so much discussion in the past few weeks.
(3)当关系代词后有插入成分时。
eg: Mary was much kinder to Jack than she was to the others, which, of course, made all the others upset.
3.指人时，关系代词只用who(m)的情况。
(1)引导非限制性定语从句时。 (2)当“介词+关系代词(宾格形式)”结构引导定语从句时。 (3)当关系代词后有插入成分时。
eg:This is my friend who,I think,is the best in our class. .
(4)在There be句型结构中，先行词指人时。
eg:There is a young lady who asks for you.
考点三：as的用法及as与which的区别
1. as引导的定语从句
(1)as常用于固定搭配中：the same. . . as，such…as，as/so…as
eg:①比较：Zhengzhou is such a wonderful city that everyone likes to visit it.
注意：第一句是定语从句，所以定从中没有出现与先行词city 意义上一致的词，
第二句是状语从句，that 后应为完整句子，所以it 不能省略。
②同样的但不是同一辆)
比较：Today he wears the same coat that he did yesterday .(同一件衣服)
(2)as常用于固定句型中：as we all / everyboby know(s), ,as is often the case,as we expect等。
eg:As we all know,Zhengzhou is an attractive city.
2.在非限制性定语从句中，as与which的区别
⑴指代整句话内容时，as可位于主句前面、中间或后面;而which一般位于主句后。另外，which还可指代一个名词或一个短语。
eg①: As we all know,Tom is a good boy.
②：China,which was founded in 1949,is becoming more and more powerful.
③：She doesn't think she is wrong,which makes her teacher angry.(which指代上文整个句子)
⑵ich没有，
eg: ①He succeeded this time,as had been expected.
②She has made great progress,which makes her parents very happy.
③As we all know,knowledge changes life.
