一、什么是定语从句？
定语从句（Relative Clause）是在复合句中，修饰名词或代词的从句。它相当于一个形容词，用来说明先行词。
例：The boy who is wearing a red hat is my brother.
（戴红帽子的那个男孩是我弟弟。）
这里的who is wearing a red hat 修饰先行词the boy。
二、引导定语从句的关系词，关系词分两类：
1. 关系代词
who（指人，作主语/宾语）
whom（指人，作宾语，正式/书面语）
whose（指人或物，表示所属关系）
which（指物/动物，作主语/宾语）
that（指人或物，作主语/宾语，口语常用）
例：The girl who sings well is my cousin. （作主语）
The man whom we met yesterday is a doctor. （作宾语）
The student whose bike was stolen is crying. （表示所属）
The book which is on the desk is mine. （作主语）
The movie that we watched last night was exciting. （作宾语）
2. 关系副词
when
（时间，= on which）
where（地点，= in/on/at which）
why（原因，= for which）
例：I still remember the day when I first met you. （时间）
This is the house where I was born. （地点）
I don’t know the reason why he was late. （原因）
三、限制性与非限制性定语从句
1. 限制性定语从句：对先行词起限定作用，不可省略，不用逗号隔开。
例：The man who is talking with my teacher is my uncle.
2. 非限制性定语从句：只是补充说明，不影响句子主干，用逗号隔开，不能用that。
例：My mother, who loves flowers very much, bought some roses.
This book, which was published in 1995, is very popular.
对比
1.The cars that are parked outside belong to the guests.停在外面的那些车属于客人。
→ 限制性（去掉会变成“这些车都属于客人” —— 不对）。
2.My car, which I bought last year, has already broken down.
→ 非限制性（去掉从句后：My car has already broken down. 主旨不变）。
四、省略关系代词
当关系代词作宾语时，可以省略。
例：The man (whom) you saw yesterday is my uncle.
The book (that) I bought is very interesting.
五、特殊用法：介词+ 关系代词
The man
to whom you spoke is my teacher. （正式语体）
The man who(m) you spoke to is my teacher. （口语常用）
先行词为最高级、不定代词、all/anything/everything/something 等时常用 that
This is the best film that I have ever seen.
Is there anything that I can do for you?
同位语从句vs 定语从句
The news that he has passed the exam is true. （同位语从句，解释 news）
The news that you told me yesterday is true. （定语从句，修饰 news）
六、经典例句总结
The girl
who is standing by the door is my sister.
The house which we visited yesterday is very old.
The boy whose father is a doctor is my classmate.
This is the reason why he failed the exam.
Do you remember the day when we first met?
This is the school where I studied ten years ago.
The teacher, who is very kind, is loved by students.
The story, which I read last week, was very moving.
