核心规则
主语从句在整个句子中充当主语，可由 that、whether、what、who、why、how 等引导。连接词的选择取决于从句意义以及它是否在从句内部充当成分。
结构与例句
That the plan succeeded surprised everyone 中 that 只连接，不作成分。What we need is more time 中 what 相当于 the thing that，在从句中作 need 的宾语。
较长主语从句常后移，用 it 作形式主语：It is clear that the two results differ；It remains uncertain whether the policy will change。
使用场景
正式说明、评价和论证常使用 It is + 形容词/名词/过去分词 + that 从句，使重要判断先出现，并避免头重脚轻。
常见错误
错误： What he said are important. → 从句整体通常按单数处理：What he said is important。
错误： If the plan will work remains unknown. → 主语从句表示“是否”通常用 Whether the plan will work。
易混知识点
that 引导的主语从句意思完整；what 引导的从句缺少名词性成分。不能说 What he will come is certain，应写 That he will come is certain。
总结
判断连接词是否需要在从句中作成分；从句过长时优先使用形式主语 it。
选择性必修 · 主语从句与形式主语
高中学习重点
常见形式主语结构包括 It is said/reported that...、It is important that...、It seems that... 和 It is a pity that...。部分表示建议、必要或命令的结构中，that 从句可用 (should) + 动词原形：It is essential that every member (should) participate。
what、whatever、whoever 等融合关系词既承担连接功能又在从句中作成分：Whatever happens will not change our decision。
小练习
- 用 it 改写：That everyone understood the rule was important.
- 填空：___ caused the failure is still being investigated.
- 填空：It is necessary that the data ___ (check) again.
参考答案
- It was important that everyone understood the rule。
What。(should) be checked。