一. 定義：定語由一個句子來充當，那麼這個句子就叫做定語從句。
eg: 其中劃線部分為定語從句。
二. 分類：分為限制性定語從句和非限制性定語從句，還有間隔性定語從句。 (限制性定語從句)
(非限制性定語從句)
( 間隔性定語從句)
注：定語從句和非限制性定語從句的區別
限制性定語從句無逗號隔開、不可缺少、起修飾限制作用;而非限制性定語從句有逗號隔開、可有可無、起補充說明作用。
三. 構成：定語從句有先行詞、引導詞和從句構成。
先行詞是定語從句所修飾限制的名詞或代詞。
引導詞是用來引導定語從句的關係代詞或關係副詞。
eg: The old town has narrow streets and small houses that are built close to each other. 此句中先行詞為：narrow streets and small houses, 引導詞為：that， 定語從句為：that are built close to each other
四.基本原則
定語從句中不能出現與先行詞在意義上相一致的詞 與book指同一物，所以要去掉。)
五.定語從句中常見考點：
考點一.關係代詞和關係副詞的辨別
1. 關係代詞that(人、物)，which(物)、who(m)(人)、whose(某人的，某物的)、as
2. 關係副詞when(表時間)、where(表地點)、why(表原因)
注：關係副詞可變為“介詞+關係代詞”結構(即：關係副詞=介詞+關係代詞)。
eg:This is the mountain village where(=in which)I stayed last month.
3. 怎樣選擇正確的'關係代詞或關係副詞 方法一：找出先行詞和定語從句中動詞，看定從中動詞與先行詞能否構成習慣搭配。能，用關係代詞;不能，用關係副詞.
Eg.I will never forget the day定從中動詞spent 與先行詞
the day 構成spent the day ,所以用關係代詞that或which或省略)
系代詞。如不缺少，定語從句所表達意思相對完整，用關係副詞。
eg:① is the factory ____ made cars . (缺少主語，所以用關係代詞that 或which，作主語不能省略)
② is the reason _______ he was late for school.(定語從句he was late for school 表
達意思相對完整，所以用關係副詞why 或介詞+關係代詞for which) 注意：當先行詞為case、position、point、situation、society、activity等抽象意義的名詞時，常用關係副詞where引導定語從句.
eg:He got himself into a dangerous situation where he may lose control of the plane .
考點二. 定從中that 與which的區別
1. 關係代詞只用that的情況。
(1)當先行詞為不定代詞(all、much、little、few、some、any、none、one、something、anything、everything等)或先行詞被不定形容詞(all、many、some、few、little等)修飾時。 that you want to say for yourself?
(2)當先行詞既有人又有物時。 that you are talking about.
(3)當先行詞含有序數詞或最高階時。
eg: ① will go to Beijing.
②(4)當先行詞被only、very、next、last等修飾時。
that I’m looking for.
(5)在who或which引導的特殊疑問句中。 that you lost?
(6)當關系代詞在定語從句中作表語時。 that it used to be.
2.關係代詞只用which的情況。
(1)引導非限制性定語從句時。
eg:He turned to be a very successful man,(2)在“介詞+關係代詞”結構中，關係代詞只用which。
eg:This is the question they have so much discussion in the past few weeks.
(3)當關系代詞後有插入成分時。
eg: Mary was much kinder to Jack than she was to the others, which, of course, made all the others upset.
3.指人時，關係代詞只用who(m)的情況。
(1)引導非限制性定語從句時。 (2)當“介詞+關係代詞(賓格形式)”結構引導定語從句時。 (3)當關系代詞後有插入成分時。
eg:This is my friend who,I think,is the best in our class. .
(4)在There be句型結構中，先行詞指人時。
eg:There is a young lady who asks for you.
考點三：as的用法及as與which的區別
1. as引導的定語從句
(1)as常用於固定搭配中：the same. . . as，such…as，as/so…as
eg:①比較：Zhengzhou is such a wonderful city that everyone likes to visit it.
注意：第一句是定語從句，所以定從中沒有出現與先行詞city 意義上一致的詞，
第二句是狀語從句，that 後應為完整句子，所以it 不能省略。
②同樣的但不是同一輛)
比較：Today he wears the same coat that he did yesterday .(同一件衣服)
(2)as常用於固定句型中：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.在非限制性定語從句中，as與which的區別
⑴指代整句話內容時，as可位於主句前面、中間或後面;而which一般位於主句後。另外，which還可指代一個名詞或一個短語。
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指代上文整個句子)
⑵ich沒有，
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.
