在英语的复杂句结构中，定语从句（Attributive Clause）扮演着至关重要的角色，用于修饰名词或代词。而在这个从句中，关系代词（Relative Pronoun）如果充当的是主语成分，那么它就是整个从句的“发动机”，决定了从句的人称和数。今天，我们就来彻底攻克这个知识点。
一、核心关系代词：Who, Which, That
当关系代词在定语从句中作主语时，我们主要使用以下三个词，它们的选择取决于先行词（Antecedent）指人还是指物。
| 关系代词 | 指代对象 | 例句 | 从句中角色 |
|---|---|---|---|
| Who | 人 | The woman who lives next door is a doctor. (住在隔壁的那个女人是位医生。) |
从句主语 (lives) |
| Which | 物/动物 | The book which is on the table is mine. (桌上的那本书是我的。) |
从句主语 (is) |
| That | 人或物 | He is the only person that understands me. (他是唯一理解我的人。) The car that was stolen has been found. (被偷的那辆车已经找到了。) |
从句主语 (understands, was stolen) |
核心要点：关系代词作主语时，绝对不能省略。因为它承担着连接主从句和充当主语的双重功能，省略后从句会失去主语，导致句子结构不完整。
二、深度分析与特殊场景
1. “That” 与 “Who/Which” 的选用
在许多情况下，that 可以替代 who 或 which，使句子听起来更简洁。但在某些正式文体或特定情况下，会有偏好。
| 优先使用 that 的情况 | 例句 |
|---|---|
| 先行词是不定代词（all, any, little, much, everything, nothing, none 等） | All that glitters is not gold. (发光的并不都是金子。) |
| 先行词被序数词或形容词最高级修饰时 | He is the best teacher that I have ever met. (他是我遇到过的最好的老师。) This is the first novel that was written by him. (这是他所写的第一本小说。) |
| 先行词同时包含人和物时 | We talked about the people and things that we remembered from school. (我们聊起了记忆中学校里的人和事。) |
2. 关系代词的“数”与主谓一致
这是一个极易出错的地方！关系代词作主语时，定语从句中的谓语动词必须和关系代词所指代的先行词在人称和数上保持一致。
- 正确: I who am your friend will help you. (先行词“I”是第一人称单数，从句谓语用“am”)
- 正确: She is one of the students who study hard. (先行词是“students”，复数，从句谓语用“study”)
- 正确: She is the only one of the students who studies hard. (先行词是“the only one”，单数，从句谓语用“studies”)
3. “As” 作关系代词
在固定搭配中，as 也可作关系代词引导定语从句，常在句首或句尾。
- As is known to all, the earth revolves around the sun. (正如众人所知，地球绕太阳转。)
- He is late again, as is often the case. (他又迟到了，这是常有事。)
三、实战演练：改错与辨析
来看看你是否真正掌握了这个知识点。
| 错误句子 | 分析与改正 |
|---|---|
| Do you know the man is standing over there? | 错误: 从句缺少主语。 改正: Do you know the man who/that is standing over there? |
| Everything which can be done has been done. | 不佳: 先行词为不定代词everything，通常优先用that。 优化: Everything that can be done has been done. |
| This is a book which describe Chinese history. | 错误: 关系代词which指代单数名词book，谓语动词应用第三人称单数。 改正: This is a book which describes Chinese history. |