爱华网⒊It + v. + to do sth.
动词不定式作主语时除动词be以外，动词take,cost， need,require,make,sound,occur 等也可用it作形式主语，而把动词不定式放在后面。例如：
It took us three years to complete the project. 我们花费三年时间才完成了这项工程。
It sounds reasonable to do it this way.听起来这样做有道理。
It needed hard work to finish the job. 要完成这项工作需要努力地工作。
It didn,t occur to me to ask him to help me. 我根本没想到要找他帮忙。
动名词作真正主语
用于这种形式的是一些特定的形容词和名词。例如：
It was nice meeting you. 见到你真高兴! (分别时用)
它与不定式作真正主语的区别在于，动名词说到的动作已经发生。例如：
It is nice to meet you. (刚刚见面时用)
it 作形式主语，动名词作真正主语主要用于下列句型：
It's +n.(名词或名词短语)/adj.(形容词)+ doing sth.
1)下列形容词后常用动名词作真正主语：nice,good,useless,hard,foolish,enjoyable,worthwhile等。例如：
It is nice talking to you. 跟你交谈真是愉快。
It's foolish behaving like that. 这样的表现是很愚蠢的。
It is useless doing that. 那样做没用。
2)下列名词或名词短语作表语时常用动名词作真正主语：use,good,fun,a waste of,job,task等。例如：
It's a waste of time doing this. 这样做是浪费时间的。
It's no good (use) doing that. 那样做没好处(没用)。
It's an awful job doing this. 做这事真是一件可怕的差事。
It's fun doing this. 做这事真有趣。
It is not an easy task doing this work. 做这工作真非易事。
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)
⒈It 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。
⒉It 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。
⒊It +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。
⒋It + 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.
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