【主语从句语法详解】主语从句是英语中一种重要的从句类型，它在句子中充当主语的角色。掌握主语从句的结构和用法，有助于提高英语写作和阅读理解能力。本文将对主语从句的基本概念、引导词、句型结构以及使用注意事项进行总结，并通过表格形式加以说明。
一、主语从句基本概念
主语从句是指由一个完整的句子作为主语的结构。它通常由连接词（如 that, whether, if, who, what, when, where, why, how 等）引导，放在句首或句中，构成整个句子的主语部分。
例如：
What he said is true.
（他说的是真的。）
“what he said” 是主语从句，作主句的主语。
二、主语从句的引导词
| 引导词 | 用法说明 | 例句 |
| that | 引导陈述性内容，不作成分，常省略 | That he passed the exam surprised everyone. |
| whether / if | 引导疑问内容，表示“是否” | Whether she will come is still unknown. |
| who | 指人，作从句中的主语或宾语 | Who broke the window is not clear. |
| what | 指物或事，作从句中的主语或宾语 | What you need is help. |
| when | 表时间 | When we will meet hasn’t been decided. |
| where | 表地点 | Where he lives is a mystery. |
| why | 表原因 | Why he left remains a question. |
| how | 表方式 | How he solved the problem is amazing. |
三、主语从句的句型结构
1. 主语从句 + be 动词 + 表语
- What I want is a good job.
- That he is late is obvious.
2. 主语从句 + 谓语动词 + 宾语
- What she did surprised us.
- Whether he comes or not doesn’t matter.
3. 主语从句 + 系动词 + 表语
- That he is honest is known to all.
- Where they live is unclear.
四、主语从句的常见表达方式
- It is + 形容词/名词 + that 主语从句
- It is important that we arrive on time.
- It is a fact that the earth is round.
- It is + 过去分词 + that 主语从句
- It is said that he is a genius.
- It is believed that the project will succeed.
五、主语从句的注意事项
1. that 引导的从句在正式语体中不能省略，但在口语中有时可以省略。
- 正确：That he failed the test was expected.
- 口语中：He failed the test was expected.（不推荐）
2. whether 和 if 的区别：
- whether 可以引导主语从句，而 if 一般不用于主语从句中。
- 正确：Whether he will join us is uncertain.
- 错误：If he will join us is uncertain.
3. 主语从句的语序：主语从句内部应使用陈述语序，而非疑问语序。
- 正确：What he wants is a car.
- 错误：What does he want is a car.
六、总结表
| 内容 | 说明 |
| 定义 | 主语从句是由一个完整句子作主语的结构 |
| 引导词 | that, whether, if, who, what, when, where, why, how 等 |
| 结构 | 主语从句 + 谓语动词 + 表语/宾语 |
| 常见句型 | It is + 形容词/名词 + that 从句；It is + 过去分词 + that 从句 |
| 注意事项 | that 不能随意省略；whether 不可替换为 if；从句内用陈述语序 |
通过以上内容的学习与练习，可以更好地理解和运用主语从句，提升英语语言表达的准确性和多样性。
