【定语从句用法】在英语语法中，定语从句是一种非常重要的句型结构，用于修饰名词或代词，提供更具体的信息。它通常由关系代词（如 who, whom, whose, which, that）或关系副词（如 when, where, why）引导。正确使用定语从句可以让句子更加清晰、准确。
一、定语从句的基本结构
定语从句一般由“先行词 + 关系词 + 从句”构成。关系词在从句中充当一定的语法成分（如主语、宾语、定语等），同时起到连接主句和从句的作用。
例如：
- The book that is on the table is mine.
（那本书是我写的。）
二、常见关系词及用法
| 关系词 | 用法说明 | 举例 |
| who | 指人，作主语或宾语 | The man who is talking to Mary is my teacher. |
| whom | 指人，作宾语（正式场合） | The girl whom you met yesterday is my sister. |
| whose | 指人或物，表示所属关系 | The student whose homework was late got a warning. |
| which | 指物，作主语或宾语 | The car which is parked outside is my father’s. |
| that | 指人或物，作主语或宾语（非正式场合常用） | The movie that we watched last night was very interesting. |
| when | 指时间，作状语 | I still remember the day when we first met. |
| where | 指地点，作状语 | This is the house where I grew up. |
| why | 指原因，作状语 | That’s the reason why she left. |
三、定语从句的分类
1. 限制性定语从句：对先行词起限定作用，不能省略，否则会影响句子意义。
- 例句：The man who is standing there is my uncle.
2. 非限制性定语从句：对先行词进行补充说明，通常用逗号隔开，即使省略也不影响句子基本意思。
- 例句：My brother, who lives in Beijing, is coming to visit us.
四、注意事项
- 当关系词在从句中作宾语时，可以省略（尤其是在口语中）。
- 例句：The book (that) I borrowed from the library is very interesting.
- 在非限制性定语从句中，不能使用 that，只能用 which 或 who/whom。
- 正确：The city, which is famous for its food, attracts many tourists.
- 如果先行词是 all, everything, something, nothing 等不定代词，通常使用 that 引导从句。
- 例句：I know everything that happened last night.
五、总结
定语从句是英语中不可或缺的语法结构，能够使表达更加丰富和准确。掌握其基本结构、关系词的用法以及不同类型的定语从句，有助于提高英语写作和口语表达能力。通过不断练习和积累，可以更加灵活地运用定语从句来增强语言的表现力。
