连接代词：who, whoever, whom, which, whichever, what, whatever
连接副词：when, where, how, why等。
That light travels in straight lines is known to all.
That she is still alive is a consolation.
(1) It is +名词+从句
It is no surprise that Bob should have won the game.
(2) It is + 形容词+从句
It is obvious that conductors and insulators are both important in industry.
(3) It is + 过去分词+从句
It was required that they should finish their homework on time.
(4) It + 不及物动词+从句
It happened that I saw him yesterday.
Whether it will do us harm remains to be seen.
Which plan is better should be discussed.
How it was done was a mystery.