一、概念
定語從句的概念，在複合句中，修飾某一名詞或代詞的從句叫定語從句。
例如： 1) The man who lives next to us is a fool.
2) You must buy me the house which is near Xiangshan.
上面兩句中的the man和the house是定語從句所修飾的詞，叫先行詞，定語從句放在先行詞的後面。
二、關係詞(連線詞)
1.關係代詞引導的定語從句
引導定語從句的關係詞有關係代詞：that, which, who (賓格whom, 所有格whose) 和關係副詞where, when, why。關係詞放在先行詞和定語從句之間，起連線作用，同時還做定語從句的一個成分。
2.關於that, which的用法注意點
1)只能用that，不用which作為定語從句關係代詞的情況
a)不定代詞，如anything, nothing, the one, all, much, few, any, little等作先行詞
There is nothing that I can do.
I mean the one that was bought yesterday.
b)先行詞有the only, the very, the same, the last修飾
This is the very book that I want to find.
The last place that I visited was the hospital.
c)先行詞為序數詞、數詞、形容詞最高階時
This is the first composition that he has written in English.
d)先行詞既有人，又有物時
He talked about the teachers and schools that he had visited.
2)不用that, 只用which的情況
a)引導非限定性定語從句時which
The tree, which is four hundred years old, is very famous here.
b)介詞後用which
We depend on the land from which we get our food.
3.關係副詞引導的定語從句
關係副詞可代替的先行詞是時間、地點或理由的名詞，在從句中作狀語。
1)先行詞是表示地點名詞時(country, school, room…)，而關係詞代替先行詞在定語從句中充當地點狀語，一般用where引導定語從句。也可以用介詞+which的結構。值得注意的是which前的介詞選擇是根據先行詞與介詞的`搭配關係而定。
Beijing is the place in which (=where) I was born.
He wants to find the place in which (=where) he lived forty years ago.
2)先行詞是表示時間名詞時(year, month, day, night…)，而關係詞代替先行詞在定語
從句中充當時間狀語，一般用when引導定語從句。也可以用介詞+which的結構。值得注意的是when前的介詞選擇是根據先行詞與介詞的搭配關係而定。
His father died that year in which (=when) he was born.
I cannot forget the first day on which (=when) my family moved into the city.
3)先行詞是the reason，而關係詞代替先行詞在定語從句中充當原因狀語，一般用why引導定語從句。也可以用for + which的結構。
Is this the reason for which (why) he refused our offer?
但是這裡要指出的是，如果介詞和Which搭配在一起並不表示以上三種意思，則我們只能保留介詞+Which的形式。如 I know English in which I wrote the love letter.
4.判斷關係代詞與關係副詞
用關係代詞，還是關係副詞完全取決於從句中的謂語動詞。及物動詞後面無賓語，就必須求用關係代詞;而不及物動詞則要求用關係副詞。
