高一英语：名词性从句——主语从句与表语从句
一、知识点讲解
名词性从句是指在句子中充当名词功能的从句。它在句中可作主语、宾语、表语或同位语。本课重点学习主语从句和表语从句。
1. 主语从句 (Subject Clause)
主语从句在复合句中充当主语，通常放在句首。引导主语从句的连接词有三类：
| 类别 | 连接词 | 功能 | 示例 |
|---|---|---|---|
| 从属连词 | that, whether | that无意义，whether表"是否" | That he passed the exam is great news. |
| 连接代词 | who, what, which, whoever, whatever | 在从句中作主语、宾语、表语 | What she said surprised everyone. |
| 连接副词 | when, where, why, how | 在从句中作状语 | Why he left remains a mystery. |
⚠️ 重要：形式主语 it 当主语从句较长时，常用 it 作形式主语，把真主语后置。
| 句型 | 公式 | 示例 |
|---|---|---|
| It + be + adj + 主语从句 | It + be + adj + that... | It is obvious that he is lying. |
| It + be + n + 主语从句 | It + be + a noun + that... | It is a pity that you can't come. |
| It + be + done + 主语从句 | It + be + p.p. + that... | It is said that the meeting has been cancelled. |
| It + seems/happens + 主语从句 | It + vi + that... | It seems that everyone agrees. |
2. 表语从句 (Predicative Clause)
表语从句位于系动词（be, seem, look, remain, feel等）之后，说明主语是什么或怎么样。
| 引导词 | 用法 | 示例 |
|---|---|---|
| that | 无意义，常可省略（正式文体中保留） | The truth is that he has never been abroad. |
| whether | 表"是否"（不可用if代替） | The question is whether we should accept the offer. |
| what/which/who | 连接代词 | That's what I want to say. |
| when/where/why/how | 连接副词 | This is why I came late. |
| because/as if | 特殊引导词 | It looks as if it is going to rain. |
二、易混辨析
1. that vs what 引导主语从句
| 引导词 | 作用 | 例句 |
|---|---|---|
| that | 只起连接作用，不充当成分，无意义 | That he succeeded surprised us. (他成功了，这让我们惊讶) |
| what | 既连接从句，又在从句中充当成分（主语/宾语/表语） | What he said surprised us. (他说的话让我们惊讶) |
2. whether vs if 的用法区别
| 使用场景 | 可用 whether | 可用 if |
|---|---|---|
| 主语从句（句首） | ✓ | ✗ |
| 表语从句 | ✓ | ✗ |
| 介词后 | ✓ | ✗ |
| 与or not连用 | ✓ | ✗（口语中可） |
| 动词后的宾语从句 | ✓ | ✓ |
三、经典例题
例1：选择引导词
_____ he will come to the party is still unknown. A. That B. Whether C. What D. If
解析： 从句"he will come to the party"结构完整，不缺成分。句意表示"是否"，且从句在句首作主语。所以选 B. Whether（if不能用于句首的主语从句）。
例2：改写句子
用it作形式主语改写： That he won the first prize made us very happy.
改写： It made us very happy that he won the first prize.
例3：完成句子
The reason for his absence is _____ he was ill.
解析： 表语从句，结构完整，用 that 引导。答案是 that。
四、趣味练习
练习1：连连看
将左边的从句与右边的句子正确搭配：
| 左 | 右 |
|---|---|
| ① That she passed the test | A. is where I was born. |
| ② Whether we go or not | B. is that practice makes perfect. |
| ③ What matters most | C. depends on the weather. |
| ④ This is | D. is no surprise. |
答案： ①-D ②-C ③-B ④-A
练习2：改错题
找出下列句子中的错误并改正： 1. If he will come is not clear. 2. The problem is if we can finish on time. 3. That we need it are more time.
参考答案： 1. If → Whether（主语从句句首不能用if） 2. if → whether（表语从句不能用if） 3. are → is（主语从句作主语被视为单数；that we need → what we need，句意更合理）
五、课后作业
一、单项选择（每题10分，共40分）
-
___ makes the book so popular is its touching story. A. That B. What C. Which D. Whether
-
It remains unknown ___ the project will be completed on time. A. that B. what C. whether D. why
-
The question is ___ we can get enough support from the public. A. that B. whether C. what D. if
-
___ is known to all is that the earth goes around the sun. A. That B. What C. It D. As
二、完成句子（每题15分，共30分）
- ___ (令人惊讶的是) he knows nothing about it.
- This is ___ (我所担心的).
三、翻译（30分）
- 我们是否应该接受这个计划，这仍然是个问题。
- 事实是他已经在这家公司工作了十年。
参考答案： 1. B 2. C 3. B 4. B 5. It is surprising that 6. what I am worried about 7. Whether we should accept the plan remains a question. 8. The fact is that he has been working in this company for ten years.
学习小贴士： 区分主语从句和表语从句的关键：主语从句在句首或由 it 替代，表语从句在系动词之后。同时记住"if不能引导主语从句和表语从句"这条硬规则！