不定式作真正主語
　　主要用于下列句型：　　1.It + is/was + adj./n.(形容詞或名詞) +to do sth.　名詞從句作真正主語
　　用it 作形式主語的結(jié)構(gòu)主要用于下列句型：　　It +謂語+名詞從句(主語從句、表語從句、同位語從句) 　　1.It is +名詞+從句:這樣的名詞有a fact,a surprise,an honor(非常榮幸),a pity,common knowledge(常識)等.例如：　　It is common knowledge that the full moon brings fair weather.皓月兆天晴,這是常識.　　It is a surprise that you gave such an answer to the question.你對這個問題作了這樣的回答,令人吃驚.　　2.It is +形容詞+從句:這樣的形容詞有natural,true,strange,necessary,important,obvious(很明顯),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.我們城市的多數(shù)人將享受免費醫(yī)療,這是一定的.　　It is probable that he has known everything.很可能他什么都已經(jīng)知道了.　　3.It +動詞+從句:這樣的動詞有seem,happen,appear,look,matter,make,strike,occur(突然想起)等.　　It appeared that he had a taste for music.看來他對音樂有一定的鑒賞力.　　It made us very happy that she was saved.她能化險為夷令我們大家很高興.　　4.It + be + 分詞+從句:這樣的分詞有 said(據(jù)說),reported(據(jù)報道),believed(人們相信),known(眾所周知),decided(已決定),proved(已證實),thought,expected,annouced,arranged,amusing,puzzling,striking等.例如：　　It is said that Li Hao has been to Europe.據(jù)說李郝去過歐洲.　　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來一只兔子,太有趣了.　　需要說明的是,大部分連接詞引導(dǎo)的主語從句都可以用 it充當(dāng)形式主語.例如：　　It remains unknown when they are going to get married.他們何時結(jié)婚依然不明.　　It says in the newspaper that the enemy has been defeated.報紙上報導(dǎo)敵人被打敗了.　　形式主語：　　It +be+(the)+adj.+for sb.+to do sth.