（4）主语从句：主语变成一个句子

构成方式：

若从句为陈述句，直接加that；

若从句为特殊疑问句：直接保留特殊疑问词，后面的句子变成陈述句语序；

若从句为一般疑问句：先把一般疑问句变成陈述句语序，然后在其前加上whether或者if；

位置：直接置于句首；

eg：That the college will take in more students is true.

Weather he will come or not hasn't been decided.

Why he isn't here is not clear to everyone.

it 做形式主语的情况：

（1）It is+名词+主语从句；

eg：It is a pity that you should have to leave.

（2）It is+形容词+主语从句；

eg：It is clear that the whole project is due to failure.

如果是It is necessary/important/strange/natural+that引导的主语从句，从句中的谓语动词一定要用should+动词原形；

eg：It is necessary that one should master the skills of operating computer.

（3）It is+过去分词+主语从句；It is said/planted/expected…

eg：It is said that President Bush will visit our school next week.

（4）It is+不及物动词+形容词表语形式+主语从句；

eg：It seemed certain that he will win the prize.