(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.