定语从句概述
定语从句的概念相当于一个修饰词，它处在一个主句中间的位置，用来限定或说明主句中某个名词的特征。定语从句根据跟随的紧密程度，可分为限定式定语从句和非限定式定语从句，前者与其修饰词紧密连接，没有逗号连接不能去掉，后者与其修饰的名词关系不紧密，有逗号分隔，即便去掉，也不影响主句的理解。
I like mixing with people who are well read.
这里Who 指代的是 'people'，who are well read就是定语从句。如果去掉从句 who are well read, 那么主句 I like mixing with people 缺乏了对什么人的描述。
She ate too much, which led to her putting on a lot of weight.
这里Which 指代 'she ate too much'，后面的从句仅仅补充说明，不影响前一个句子（主句）的完整性。
定语从句的好处是能够避免不必要的重复，将两个简单句合并为一个复合句。
I bought a new car. The car has a sun roof.
合并成I bought a new car, which has a sun roof. 后面是个非限定性定语从句。
I tried to write the whole essay in one hour. Writing the whole essay in one hour wasn't possible.
合并成I tried to write the whole essay in one hour, which wasn't possible.
上面是定语从句修饰主句中的宾语情况，实际上还可以修饰主语。
The students (主语) who studied the hardest got the highest scores in the test.
Canberra (主语), which is the capital of Australia, is a relatively new city.
下面两个句子你能分清哪个是限定性，哪个是非限定性定语从句吗？
She is the person who stole my bag.
The zoo, which was built seven years ago, is one of the best in the country.
怎样识别定语从句
定语从句有下列关系代词引导：
- who
- that
- which
- whose
- whom
- what
例如：
Marie Curie is the woman who discovered radium.This is the house which Jack built.
This is George, whose brother went to school with me.
I had an uncle in Germany, from who(m) I inherited a bit of money.
I didn't know the uncle that I inherited the money from.
但有时我们可以省略关系代词，如果它是动词的宾语，例如：
They’re the people she met at Jon’s party.
Here are some cells the researcher has identified.
关系副词也可以引导定语从句:
- where
- when
- why
例如：
Do you remember the year when we got married?
Stratford-upon-Avon is the town where Shakespeare was born.
We understand the reason why you did not want to come.
定语从句的动词形态要与先行词单复数统一
定语从句动词的变化要与修饰的先行词的单复数一致：
Here are some cells which/that show abnormality.
Here is the cell which/that shows abnormality.
定语从句的简化
定语从句可以用分词结构或短语替代，如下面的例句：
- The book which is on the table is mine.等同
- The book on the table is mine.
- John, who had taken his medicine, laid down and slept.等同
- John, having taken his medicine, laid down and slept.
何时定语从句用that
有一种说法，强调句后面的定语从句要用that。在先行词为前有most, only, anyone, the first, the best, the only, 或no one, none, someone时，定语从句要用that, 不能用who, 也许这是中国人学语法的死要求吧，用who在口语中也不一定错误。但that绝对不能用在非限定性定语从句。
The most impartial critic that could be found.The only man that I know.Any one that knows anything knows this.It was you that said so.Who is it that talks about moral geography?
定语从句中的介词
定语从句的关系代词有时候会和介词有搭配，如果介词不放在后面就可以放在关系代词的前面。如下面的定语从句中的介词不能没有。
The chair that I am sitting in isn’t comfortable.
The person who I live with is my cousin.
The school which I go to is in Oregon.
如果句子比较正规，介词会放在关系代词的前面：
The house in which Mozart was born is now a museum.
Mr Pierce, to whom I talked just now, sends you his regards.
New York is a city to which I have traveled many times.
This is the problem for which there is no perfect solution.
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