主语从句
从句充当句子的主语，常用 it 作形式主语放到后面。
What he said is true. / It is + 形容词 + that …
要点
- 连接词有 that、whether、what、who、when、why、how 等。
- that 引导主语从句时不能省略。
- 常用 it 作形式主语，把从句挪到后面：It is important that …
- 主语从句作主语时，谓语一般用单数。
- whether 可以放在句首，if 不能。
- It is + 形容词 + that 从句里若说的是"重要、必要、建议"，动词走 should + 原形或裸原形：It is essential that every ticket be checked.（英式常写 should be checked）—— 这一式讲的是语气，与时态退格无关。
- when / how / whether 领的主语从句也能压成"疑问词 + to do"，句子才不头重：When to start is still a question. —— 与宾语位置的"疑问词 + to do"是同一条路（见「宾语从句的简化」）。
例句
It is a pity that you cannot come to the party on Sunday.
你周日来不了聚会，真可惜。
What he said is true.
他说的是真的。
Whether we will go depends on the weather.
我们去不去取决于天气。
It is important that everyone should be on time.
每个人都准时很重要。
When to hold the meeting has not been decided.
什么时候开会还没定下来。
It is obvious that he has forgotten again.
显然他又忘了。
易错提示
- 句首的 that 不能省，这是主语从句与宾语从句不一样的地方。
- whether 能引导主语从句，if 不能。
- 句首挂 that 从句偏书面也显头重，日常几乎都走 it 那一式：That he forgot is obvious. 改成 It is obvious that he forgot. —— 两式都对，选哪个只为顺口。