在复合句中，修饰某一名词或代词的从句叫做定语从句。被修饰的名词或代词叫做先行词。引导定语从句的词叫做关系词。
关系词分为两类：
· 关系代词：who, whom, whose, which, that
· 关系副词：when, where, why
关系词在定语从句中充当一定成分（主语、宾语、定语、状语等），同时起连接主句和从句的作用。
The man who is standing there is my uncle.
先行词：the man；关系词：who（在从句中作主语）
① 连接功能——把主句和从句连接起来。
② 成分功能——在从句中充当某个句子成分（主语、宾语、定语、状语）。这个功能决定了用哪个关系词。
关系代词 vs 关系副词的"选择依据"：
👉 判断流程：① 找出先行词；② 分析从句缺什么成分；③ 根据成分选择关系词。
注意：判断用关系代词还是关系副词，不是看先行词是"时间/地点/原因"，而是看关系词在从句中作什么成分。
| 关系词 | 先行词 | 在从句中充当的成分 | 例句 |
|---|---|---|---|
| who | 人 | 主语 / 宾语 | The man who came is my uncle. |
| whom | 人 | 宾语 | The man whom I met is my uncle. |
| whose | 人或物 | 定语 | The boy whose father is a doctor is my friend. |
| which | 物 | 主语 / 宾语 | The book which I bought is interesting. |
| that | 人或物 | 主语 / 宾语 | The man that came is my uncle. |
| when | 时间 | 时间状语 | I remember the day when I met her. |
| where | 地点 | 地点状语 | This is the house where I lived. |
| why | 原因（reason） | 原因状语 | The reason why he was late is unknown. |
⭐ 关键提醒：判断用关系代词还是关系副词，只看"关系词在从句中充当什么成分"，不看先行词是什么。
who：先行词指人，在从句中作主语（口语中也可作宾语）。
The man who is speaking is my teacher.
whom：先行词指人，在从句中作宾语（正式用法）。
The man whom you met yesterday is my teacher.
whose：先行词指人或物，在从句中作定语，表示"……的"。
The boy whose father is a doctor is my friend.
I live in a house whose windows face south.
which：先行词指物，在从句中作主语或宾语。
The book which is on the desk is mine.
The book which I bought yesterday is interesting.
that：先行词指人或物，在从句中作主语或宾语。that 在限制性定语从句中可替换 who/whom/which。
① 关系代词作宾语时可以省略：
The book (that/which) I bought yesterday is interesting.
that/which 作 bought 的宾语，可以省略。
② 关系代词作主语时不能省略：
The man who is standing there is my uncle.
who 作 is standing 的主语，不能省略。
③ 关系代词 whom 的特殊用法：
④ that 与 who/which 的选择：
在限制性定语从句中，that 可以指人也可以指物，用法最广。但下列情况只能用 that：
👉 口诀：不定代词、序数词、最高级、only/the very——遇到这些只用 that。
The book ____ I bought yesterday is very interesting.
📌 要点：作宾语时，关系代词可省略，这是最常见的一种"省略"现象。
The girl ____ mother is a teacher is my classmate.
📌 口诀：看到"……的"，就想到 whose。
The book which I bought it yesterday is interesting.
📌 要点：关系代词已经在从句中充当了成分，从句中不能再出现指代先行词的代词。
when：先行词是表示时间的名词（day, time, year, moment 等），在从句中作时间状语。
I still remember the day when I first came to this school.
where：先行词是表示地点的名词（place, house, city, school 等），在从句中作地点状语。
This is the house where I was born.
why：先行词是 reason，在从句中作原因状语。
The reason why he was late is unknown.
关系副词在意义上等于"介词 + which"：
| 关系副词 | 等价形式 | 例句 |
|---|---|---|
| when | on/in/at + which | the day when = the day on which |
| where | in/at/on + which | the house where = the house in which |
| why | for + which | the reason why = the reason for which |
⚠️ 核心判断方法（最容易出错的地方）：
判断用关系代词还是关系副词，不看先行词是什么，而是看关系词在从句中作什么成分：
对比示例：
① This is the house which I bought last year.（which 作 bought 的宾语）
② This is the house where I lived last year.（where 作 lived 的地点状语，lived 是不及物动词，不需要宾语）
记忆口诀：句意完整用关系副词，句意残缺用关系代词。
This is the house ____ I bought last year.
📌 口诀：看先行词不如看从句——从句缺什么，就补什么。
He got into a situation ____ he couldn't solve the problem.
📌 要点：where 的先行词可以是抽象名词（表示"情境、阶段"）。
The reason ____ he gave was not true.
📌 要点：the reason 后的关系词不一定是 why，要看从句缺什么成分。
限制性定语从句：是先行词不可缺少的修饰成分，去掉后主句意思不完整。不用逗号与主句隔开。
The students who passed the exam were praised.
（通过考试的学生受到表扬——不是全部学生，只表扬通过考试的）
非限制性定语从句：是对先行词的补充说明，去掉后主句意思仍然完整。用逗号与主句隔开。
The students, who passed the exam, were praised.
（学生们都通过了考试，全体受到表扬）
非限制性定语从句的特殊规则：
· 不能用 that 引导；
· 关系代词不能省略；
· which 可以指代整个主句的内容。
| 对比项 | 限制性 | 非限制性 |
|---|---|---|
| 逗号 | 无 | 有逗号隔开 |
| that | 可以用 | 不能用 |
| 关系代词省略 | 作宾语时可省略 | 不能省略 |
| which 的指代 | 只能指代先行词 | 可指代先行词或整个主句 |
| 句意 | 是不可缺少的限制 | 是补充说明 |
非限制性定语从句中 which 指代整个主句的例子：
He was late again, which made his boss angry.
which 指代"他又迟到了"整个事件，在从句中作主语。
He didn't pass the exam, which we had expected.
which 指代"他没通过考试"整个事件，在从句中作 expected 的宾语。
👉 口诀：非限制性定语从句三不用——不用 that，不省略，which 可代整句。
My mother, ____ is a teacher, works in this school.
📌 口诀：非限制性定语从句——不用 that，不能省略。
The book, I bought yesterday, is very interesting.
📌 要点：非限制性定语从句中关系词不能省略。
He was late, ____ made his teacher angry.（选项：that / which）
📌 口诀：代整句，用 which，前面必须有逗号。
在正式文体中，定语从句的关系代词可以和介词一起使用，构成 "介词 + which/whom" 结构。
This is the house in which I was born.
The person to whom I spoke is my teacher.
使用规则：
· 介词后指人只能用 whom，不能用 who 或 that；
· 介词后指物只能用 which，不能用 that；
· 介词不能省略（除非把介词移到从句末尾）。
① 依据一：从句中的固定搭配——介词是从句中某个动词、形容词或名词的固定搭配。
This is the book on which I spent 50 yuan.
（spend money on sth. 是固定搭配，所以介词用 on）
② 依据二：先行词的习惯搭配——介词与先行词形成常见的语义搭配。
I still remember the day on which we first met.
（"在某一天"用 on the day，所以介词用 on）
介词 + 关系代词的两种形式：
👉 要点：关系副词 when/where/why 本质上是"介词 + which"的等价形式。所以：
The person to ____ I spoke is my teacher.
📌 口诀：介词后面紧跟 whom（人）或 which（物）。
This is the book which I spent 50 yuan.
📌 要点：介词跟着从句的固定搭配走，不能遗漏。
The reason for which he was late is unknown.
📌 要点：when = on/in/at which，where = in/at/on which，why = for which。
The book which I bought it yesterday is very interesting.
I live in a house. Its windows face south.
📌 要点：whose + 名词 = the + 名词 + of which = of which + the + 名词。
____ we all know, the earth moves around the sun.
📌 口诀：置于句首用 As，置于句尾可 As 可 which。
I don't like the way ____ he talks to his parents.
📌 口诀：the way 后三种选——that / in which / 不填，就是不能用 how。
It was in the park ____ I met him.
📌 要点：看去掉 It is/was 后句子是否完整。
This is the factory ____ produces cars.
📌 口诀：先看从句，再看先行词。
He is the only one of the students who ____ (be) late.
📌 口诀：the only one of... 后用单数，one of... 后用复数。
The boys ____ are playing football are my classmates.
上海卷对定语从句的考查主要集中在：
定语从句每年为上海卷直接贡献 3—5 分，且是中译英提分的关键语法点。