主语从句
用作主语的从句叫主语从句。
(That the earth goes around the sun) is known to all.
(How this happened) is not clear to anyone.
(Whoever comes) is welcome.
(When we shall have our sports meeting) is still a question.
(Whether he will come today) is still unknown.
注意：
1. 主语从句是特殊疑问句，应该用特殊疑问词加陈述句语序。
2. that引导的主语从句放在句首，不可省略that。
3. 主语从句常用it作形式主语，真正的主语放在后面，这时that可省略。
It's a pity (that) you missed such a good opportunity.
It's still a question when we shall buy the new equipment.
It's still unknown whether he will come or not.
4. It is + P.P.+ that ; It + Vi + that，It作形式主语。
It is said that the president will visit our company.
It seems that he has caught a cold.