一、先建立一张"地图"
初中阶段你已经学过定语从句：它修饰名词，相当于一个形容词。高一必修一、必修二的重点转向名词性从句——从句在句子里充当的是名词能充当的成分。名词能做什么，名词性从句就能做什么：
名词在句中可作主语、宾语、表语、同位语，于是就有四种从句：主语从句、宾语从句、表语从句、同位语从句。
判断一个从句是哪一种，只看两件事：从句在主句中充当什么成分。不要从句意去猜。
例： That he passed the exam surprised us.（主语从句——整个从句是 surprised 的主语） I know that he passed the exam.（宾语从句——从句是 know 的宾语） The fact is that he passed the exam.（表语从句——从句在系动词 is 之后） The fact that he passed the exam is surprising.（同位语从句——从句解释 fact 的具体内容）
同一个 that 从句，位置变一变，名称就变。这就是为什么语法书要求你先找"主干"。
二、引导词及两个判断标准
名词性从句的引导词分四类：
- that：无词义，在从句中不作任何成分，只起连接作用。口语中宾语从句的 that 可省略，主语从句、表语从句、同位语从句的 that 一般不省。
- whether / if：表示"是否"。whether 可用于所有四种从句；if 只能用于宾语从句，且不能跟 or not 紧挨，不能用于介词后。
- 连接代词：what（什么／……的东西）、who、whom、whose、which、whatever、whoever。它们在从句中作主语、宾语、定语等成分，这是它们与 that 的根本区别。
- 连接副词：when、where、why、how，在从句中作状语。
判断题的黄金标准只有一条：看从句里缺不缺成分。
缺主语或宾语 → 用 what / who / which（连接代词）。 不缺任何成分、意思完整 → 用 that（陈述）或 whether / if（疑问）。 缺状语（时间、地点、原因、方式）→ 用 when / where / why / how。
对比： What he said is true.（said 缺宾语 → what） That he said so is true.（said so 已完整 → that） This is where we met.（met 后不缺宾语，缺地点状语 → where）
三、四种从句逐个体检
1. 主语从句
That she won the prize made her parents proud. What matters most is your attitude.
主语从句常后置，用 it 作形式主语，这是考试重灾区： It is said that… 据说…… It is reported that… 据报道…… It is important / necessary / natural that…（从句用 should＋动词原形，should 可省） It doesn't matter whether he comes or not.
注意：主语从句作主语时，谓语一般用单数。What we need is more time.（即便从句里是复数概念，主句谓语也常用单数。）
2. 宾语从句
I don't know whether he will come. She asked me where I had been. I believe (that) he is honest.
三条纪律： ① 语序：宾语从句一律用陈述语序。"Do you know where he lives?" 不能说 "where does he live"。 ② 时态呼应：主句是过去时，从句用相应的过去时态；但从句表示客观真理时仍用一般现在时。The teacher told us that the earth moves around the sun. ③ 介词后的从句用 whether 不用 if，且 that 一般不省：It depends on whether we have enough money.
3. 表语从句
The question is whether we can finish on time. That's why he was late. The reason why he failed is that he was careless.
易错点：The reason is that…（不用 because）。虽然口语里能听到 "The reason is because…"，但书面考试判错。
4. 同位语从句
常跟的同位名词：fact、news、idea、hope、belief、doubt、possibility、promise、evidence、word（消息）等。从句说明这些名词的具体内容。
The news that our team won excited everyone. There is no doubt that he will succeed.
与定语从句的区别（最高频的辨析题）： 同位语从句：that 不作成分，从句＝名词的内容。The news that we won the game is true.（that 后不缺成分） 定语从句：that 作成分（主语或宾语），从句＝对名词的修饰限定。The news that he told me is true.（told 缺宾语，that 是宾语）
一个动作就能分辨：把 that 后面的部分单独拿出来看，意思完整 → 同位语；缺宾语 → 定语从句。
四、五个高频陷阱
- that 与 what 混用。记住：that 不作成分，what 必作成分，且 what＝the thing(s) that。 ✗ I don't understand that he means. ✓ I don't understand what he means.
- 从句用疑问语序。 ✗ Tell me what did he say. ✓ Tell me what he said.
- if 用在主语从句或介词后。 ✗ If he will come is unknown. → ✓ Whether he will come is unknown.
- 多余的 it。主语从句已后置时不要再加形式宾语：✗ He made it clear that… 这句其实是对的（it 是形式宾语，that 从句是真正宾语），但 ✗ That he is honest is it true 这类混搭必须避免。核心原则：it 只在从句后置时代替它占位。
- whether or not 的位置。"whether or not he agrees" 与 "whether he agrees or not" 都对；"if or not" 一定错。
五、练习
一、用适当的引导词填空。 1. _ he will attend the meeting is still unknown. 2. I can't imagine made him change his mind. 3. The reason he gave was _ he had missed the bus. 4. There is a possibility the project will be delayed. 5. It all depends on ____ the weather is fine tomorrow.
二、改错。 6. Do you know where does he work? 7. That you said at the meeting surprised us all. 8. The reason is because he didn't prepare well. 9. I doubt if or not she will come.
三、翻译。 10. 我们是否需要更多时间还是个问题。 11. 他迟到的事实并不能说明他不认真。 12. 这正是我所担心的。
参考答案： 1. Whether（主语从句，表"是否"，句首不用 if） 2. what（made 缺主语，指"什么"） 3. that（表语从句，说明 reason 的内容；书面语不用 because） 4. that（同位语从句，说明 possibility 的内容） 5. whether（介词 on 后用 whether，不用 if） 6. where he works（宾语从句用陈述语序） 7. What you said（said 缺宾语） 8. The reason is that he didn't prepare well. 9. I doubt whether she will come or not.（if 不能与 or not 紧连） 10. Whether we need more time is still a question. / It is still a question whether we need more time. 11. The fact that he was late doesn't mean he isn't serious.（同位语从句，that 不作成分） 12. This is what I was worried about. / This is exactly what worried me.
六、复习建议
名词性从句的所有题目，归根到底只问两个问题：从句在主句里做什么成分？从句内部缺什么成分？把这两句话写在笔记本第一页，每做一道题都用笔标出答案，两周之后这类题的正确率会稳定在 90% 以上。语法不是靠语感蒙的，是靠结构判的。