定语从句用法
在句中作定语的从句称为定语从句。定语从句通常位于它所修饰的名词或代词之后，这种名词或代词称为先行词。定语从句由关系代词或关系副词引导，关系词除了起引导作用外，还在从句中指代前面的先行词，并在从句中担任一定的成分。
1.关系代词who, whom, whose, which, that引导的定语从句
1)who, whom代替人。who是主格，在从句中作主语；whom是宾格，在从句中作宾语，在限制性定语从句中可以省略。
The man who talked to you just now is an engineer.
刚才与你谈话的那人是工程师。
She is the singer (whom) I met last week.
她就是我上周遇见的那位歌唱家。
2)whose多指人，也可指物，指物时可与of which互换使用。whose是所有格，在从句中作定语。
Madam Curie is a great scientist whose name is known all over the world.
居里夫人是一位举世闻名的伟大科学家。（指人）
Please pass me the book whose cover is green.
请递给我那本绿色封面的书。（指物）
当whose代替物时，就相当于of which。上面这句可以改为：
Please pass me the book of which the cover is green.
3)which代替物，在从句中作主语或宾语。作宾语时可以省略。
The car which was stolen has been found.
偷走的车已被找到了。
The story (which) we heard yesterday is very moving.
昨天我们听到的那个故事非常感人。
4)that代替人或物，在从句中作主语或宾语，作宾语时可以省略。
He is the only one among us that knows English.
他是我们当中唯一懂英语的人。
These are the pictures (that) I took in Shanghai.
这些是我在上海拍的照片。
5)通常只用关系代词that，不用which, who，或whom的几种情况
①先行词为all, something, everything, anything, nothing, little, much, none, few等不定代词时，只能用that。
Professor Li has told us something that we should do in the summer vacation.
李教授已经告诉我们暑假应该做的事。
Much that I learned in the book is very useful.
我在那本书里学到的很多东西是非常有用的。
②先行词前有序数词或形容词最高级修饰时，只能用that
The first thing that they should do is to study hard.
他们应该做的第一件事就是努力学习。
This is the most interesting film that I have seen.
这是我看过的最有趣的一部电影。
③先行词被some, no, every, any, only, very, much等限定词修饰时，只能用that
This is the very book that I have been looking for.
这正是我在寻找的书。
There is no person that doesn’t make mistakes.
没有人不犯错误。
④先行词既有人又有物时，只能用that
They are talking about the teachers and schools that they have visited.
他们正在谈论他们访问过的老师和学院。
⑤主句是以who或which开头的特殊疑问句时，只能用that
Who is the person that is standing over there?
站在那边的那个人是谁？
Which is the car that has overtaken us?
超过我们的是哪辆车？
2.关系副词when, where, why引导的定语从句
1)when作状语，引导的从句修饰表示时间的先行词。
I shall never forget the day when /on which I entered the university.
我永远不会忘记上大学的那一天。
2)where作状语，引导的从句修饰表示地点的先行词。
That is the company where /in which I worked three years ago.
那就是我三年前工作过的那家公司。
3)why作状语，引导的从句修饰表示原因的先行词。
We know the reason why /for which he was angry.
3.限制性定语从句和非限制性定语从句
1)限制性定语从句
限制性定语从句和先行词有着内在的不可分的联系，不能缺少，否则影响全句的意义；从句前不用逗号。
The man who came yesterday is Mary.
昨天来访的那个人是玛丽。
This is the house which we bought last month.
这是我们上个月买的那栋房子。
2)非限制性定语从句
非限制性定语从句和先行词之间是一种松散的修饰关系，只起补充说明的作用，即使省略了也不影响主句意义的完整；从句前有逗号分开，从句中的关系代词不能省略。
The house, which we bought last month, is very nice.
这栋房子很漂亮，是我们上个月买的。
My brother, who works abroad, is coming next week.
我哥哥在国外工作，他下周将回来。
He spoke confidently, which impressed me deeply.
他说话非常自信，那给我留下了很深的印象。