一、定语从句的概念
修饰名词或代词的从句称为定语从句，被定语从句修饰的名词或代词叫先行词，定语从句要放在先行词之后，定语从句由关系代词或关系副词来引导，关系代词在定语从句中可作主语、宾语和定语，关系副词可用作状语。 如:Do you know the young man who came to see your brother this morning?
今天早上来看你哥哥的那个年轻人你认识吗？
This is the factory where his father works.
这就是他父亲在里面工作的那家工厂。
1.引导定语从句的关系代词
引导定语从句的关系代词有 who, whom, whose, which ,that, 其用法如下:
(1) 先行词是表示“人”的名词时，作主语用 who/that，做宾语用 whom/that或省略，作定语用whose。
如 The girl who/that is singing there is his sister.
在那儿唱歌的女孩是他妹妹。
Do you know the student (whom/that) the teacher is speaking to?
你认识老师和他谈话的那个学生吗？
Is anyone in your class whose family is in the northwest?
你们班有谁家在西北吗？
(2) 在先行词是表示“物”的名词时，作主语用 which/that,作宾语用 which/that 或省略，作定语用 whose。
如 A plane is a machine which/that can fly.
飞机是一种会飞的机器。
The pen (which/that) I bought yesterday writes well.
昨天我买的那只钢笔很好写。
Our two countries are neighbours whose friendship is of long standing.
我们两国是有着悠久友谊的邻邦。
引导定语从句的关系副词有 when, where, why。
如:October lst, 1949 was the day when the People's Republic of China was founded.
1949 年10 月1 日是中华人民共和国成立的日子。
This is the village where I lived when I was a child.
这是小时候我生活过的村庄。
I don't know the reason why she looks unhappy.
我不知道他今天不高兴的原因是什么。
定语从句中，使用哪个关系词，取决于该关系词在定语从句中的作用。试比较：
The factory where his father works is in the west of the city.
他父亲在里面工作的那家工厂位于城市西部。(works 是不及物动词，关系副词 where 在句中作状语)
The factory which (that) we visited last week is in the west of the city.
上周我们访问过的那家工厂位于城市西部。(which或 that 在句中作及物动词 visited 的宾语)
3.关系代词 that 和 which 的用法比较
如果先行词由序数词、形容词最高级或 first, last, next, only, very 等词修饰，定语从句须由 that 引导。
如:This is the best English film that I have ever seen.
这是我看过的最好的一部英语影片。
The first thing that we should do is to work out a plan.
我们要做的第一件事是制定一份计划。
先行词是不定代词时，定语从句也须由 that 引导。
如:Everything that he said seemed quite reasonable.
他说的话似乎都很有道理。
介词后只能用 which。
如:The earth on which we live is round.
我们在上面生活的地球是圆的。
This is the classroom in which we study.
这就是我们在里面学习的教室。
但如果把介词移到句尾，则要用 that。
如:The earth that we live on is round.
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