(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.
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.