导读：关系代词是连接主句和从句的桥梁，而that则是其中用法最复杂、规则最特殊的一个。本文将深入解析它的使用场景和限制条件，帮助学习者彻底掌握这一语法难点。
关系代词that用于引导限定性定语从句，代替先行词（人、物或概念），同时在从句中充当主语、宾语或表语成分。与which/who不同，that带有更强的限制性和确定性。
| 语法功能 | 例句 | 说明 |
|---|---|---|
| 作主语 | The book that is on the table is mine. | that代替book，在从句中作主语 |
| 作宾语 | This is the car that I bought yesterday. | that代替car，在从句中作bought的宾语 |
| 作表语 | He is not the fool that you thought him. | that在从句中作表语 |
| 场景 | 例句 | 解析 |
|---|---|---|
| 先行词为不定代词 | All that glitters is not gold. | all为不定代词，必须用that |
| 先行词被序数词修饰 | The first lesson that I learned was unforgettable. | first为序数词，必须用that |
| 先行词被最高级修饰 | This is the best movie that I have ever seen. | best为最高级，必须用that |
| 先行词被the only/very/same修饰 | He is the only person that understands me. | only修饰person，必须用that |
| 先行词既有人又有物 | They talked about the persons and things that they remembered. | 同时指人和物，必须用that |
| 主句是以who/which开头的疑问句 | Who is the man that is standing there? | 避免重复"who"，改用that |
| 限制情况 | 错误示例 | 正确形式 |
|---|---|---|
| 非限定性定语从句 | My car, that I bought last year, broke down. | My car, which I bought last year, broke down. |
| 介词后接关系代词 | This is the house in that I lived. | This is the house in which I lived. |
| 先行词为"those+人" | Those that want to go please sign up. | Those who want to go please sign up. |
当关系代词在从句中作宾语时，that可以省略，而which/who在正式文体中通常不省略：
在某些固定句型中，必须使用that：
| 句子 | 选项 | 答案与解析 |
|---|---|---|
| Is this the best novel ___ you have read? | A. which B. that C. who | B. that - 先行词被最高级best修饰 |
| The woman ___ spoke to you is my aunt. | A. which B. that C. whom | B. that - 指人，在从句中作主语 |
总结：关系代词that的用法看似复杂，但掌握其核心规则后就能灵活运用。记住六大必须使用that的场景和三种不能使用的情况，结合大量练习，就能彻底攻克这一语法难点。
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