什么情况下能用形式主语it呢

不定式作真正主语
　　主要用于下列句型：　　1.It + is/was + adj./n.(形容词或名词) +to do sth.　名词从句作真正主语
　　用it 作形式主语的结构主要用于下列句型：　　It +谓语+名词从句(主语从句、表语从句、同位语从句) 　　1.It is +名词+从句:这样的名词有a fact,a surprise,an honor(非常荣幸),a pity,common knowledge(常识)等.例如：　　It is common knowledge that the full moon brings fair weather.皓月兆天晴,这是常识.　　It is a surprise that you gave such an answer to the question.你对这个问题作了这样的回答,令人吃惊.　　2.It is +形容词+从句:这样的形容词有natural,true,strange,necessary,important,obvious(很明显),certain,wonderful,funny,possible,likely,probable等.例如：　　It is certain that free medical care will be given to most people in our city.我们城市的多数人将享受免费医疗,这是一定的.　　It is probable that he has known everything.很可能他什么都已经知道了.　　3.It +动词+从句:这样的动词有seem,happen,appear,look,matter,make,strike,occur(突然想起)等.　　It appeared that he had a taste for music.看来他对音乐有一定的鉴赏力.　　It made us very happy that she was saved.她能化险为夷令我们大家很高兴.　　4.It + be + 分词+从句:这样的分词有 said(据说),reported(据报道),believed(人们相信),known(众所周知),decided(已决定),proved(已证实),thought,expected,annouced,arranged,amusing,puzzling,striking等.例如：　　It is said that Li Hao has been to Europe.据说李郝去过欧洲.　　It was amusing that a rabbit ran out of the empty box on the desk.桌上的空盒子里竟然跑出来一只兔子,太有趣了.　　需要说明的是,大部分连接词引导的主语从句都可以用 it充当形式主语.例如：　　It remains unknown when they are going to get married.他们何时结婚依然不明.　　It says in the newspaper that the enemy has been defeated.报纸上报导敌人被打败了.　　形式主语：　　It +be+(the)+adj.+for sb.+to do sth.