it作形式主语是什么?
动词不定式、动名词短语起、从句在句子中起主语作用，而这一部分用词较多时，可以用it作为形式主语，放在句首代表其后所说的事实上的、真正的主语，而把真正的主语放在后面。
1、It +谓语＋动词不定式。It作形式主语，动词不定式作真正的主语。例如：It is difficult to climb a mountain.爬山是很艰难的。It’s a good habit to do morning exercises.作早操是个好习惯。It’s important to do proper memory work in the study of a foreign language.在学习外语时适当地做一些有助于记忆的练习是很重要的。It is right to do so.这样做是对的。
2、It＋谓语＋动名词短语。It作形式主语，动名词短语作真正的主语。例如：It is dangerous playing with fire.玩火是危险的。It is no use learning without thinking.学而不思则罔。It’s useless arguing with a silly boy.和笨孩子争论是没有用的。
3、It＋谓语＋名词性从句。It作形式主语，以that引导的名词性从句是真正的主语。例如：It is a pity that you didn’t see such a good film.你没看这么好的电影，真可惜。It is certain that we shall succeed.我们一定会成功。
It is strange that nobody knows where he lives.真奇怪，谁也不知道他住在哪里。（It is strange that…后面可用虚拟语气。如：It is strange that he should have made such a mistake.他居然犯了这样一个错误，真奇怪。It is strange that nobody should know where he lives.居然没有人知道他住在哪里，真奇怪。）
It is said that the plane will take off at ten tomorrow morning.据说飞机明天上午十点起飞。
