标题：形式主语
用法一：it作逻辑主语
综述形式主语it 作为形式主语的it并无实际语义，只是为满足语法上的需要，避免句子头重脚轻，它代替的是句子的逻辑主语，概括起来会有如下情况：
不定式作逻辑主语如：It is very kind of you to give me a hand in time. 
It is my pleasure to address the meeting. 
It was pretty hard for him to bring up the child on his own.
从句作逻辑主语如：It so happened that the tickets were sold out. 　
It is extremely obvious that she has been lying about her identity. 
It suddenly occurred to me that the message must have been hidden 
between the layers of the briefcase.
动名词短语作逻辑主语这类句子远不如前面两种出现率高，主要在（no good,waste,useless,no use,
dangerous）等词语的后面用 
如：It is no use reasoning with him. 
It is no good reading in dim light. 
作形式主语的代词只能用it ，不能用that，this等词。例如:
1） It is easier to lose friends than to make friends. 交朋友难而失去朋友比较容易。
2） It is important for modern young people to master at least two foreign languages. 掌握至少两门外语对于当代年轻人来说是很重要的。
以上两句中的it是形式主语，真正的主语是后面的不定式短语。
3） It is considered no good reciting without understanding. 不理解的背诵被认为是没有好处的。
4） It is no use crying over spilt milk. 覆水难收。
以上两句中的it是形式主语，真正的主语是V-ing形式。
5） It is a fact that English is being accepted as an international language.英语作为一门国际语言正日益为人们所接受，这是不争的事实。（NMET,95）
6） It depends on the weather whether we will go. 我们去不去得看天气行事。（其中的whether 不能换成if，因为if不能引导主语从句。）
以上两句中真正的主语是名词性从句。
用法二：it作形式主语
it 作形式主语，主要分为下列三种情况：不定式作主语、动名词作主语 和 名词从句作主语。具体分析如下：
不定式作真正主语主要用于下列句型：
⒈It + is/was + adj./n. (形容词或名词） +to do sth. 
A. 名词作表语。主要有a pity,a pleasure,an idea等。例如：
In fact________is a hard job for the police to keep order in an important football match. （2001上海高考题）
A. this B. that 
C. there D. it （答案为D)
再如：It is a great pleasure to do this. 这样做多好啊。
It is a good idea to think this way. 这样考虑问题是个好办法。
除be外，其他动词也有跟名词的情况。例如：
It seems a pity to waste the food. 浪费这些食物真可惜。
It requires efforts to master a foreign language. 掌握一门外语需要种种努力。
B. 形容词或形容词短语作表语。分两种情况：
1） 下列形容词：kind,good,nice,clever,wrong,right,foolish,wise,unwise,stupid,rude,careless,cruel,brave,naughty,polite,selfish等词作表语时，不定式前常可加一个由of引起的短语，来说明不定式指的是谁的情况。形容词表示不定式逻辑主语的特征，即形容词用来描述不定式行为者的性格、品行等。例如：
It was foolish of you to leave school.你中途退学，真是傻瓜。
It was brave of her to speak out in public. 她能当众大声讲话，真勇敢。
这种of sb. to do sth. 句型，of 的宾语可以作句子的主语。上面两个句子可以改写为：
You were foolish to leave school.
She was brave to speak out in public.
2） 其他形容词作表语，常见的形容词有：necessary,hard， important,difficult,easy,possible，common等。例如：
It is necessary to use a short-wave radio to pick up the programmes. 必须用短波收音机才能收到节目。
如果要说明不定式表示的动作是谁做的，可以在不定式前加for + 名词或代词+ to do sth.(for 后的宾语不能作句子的主语）。名词或代词作不定式的逻辑主语，即表示不定式所指动作的执行者。在此句型中，形容词只对不定式行为者作某种描述，而不涉及其品行。例如：
It is not hard for one to do a bit of good. 对于一个人来说做点好事并不难。
It,s common for leaves to fall from the trees in the fall. 秋天树叶从树上落下是件常事。
需要注意的是：动词不定式作主语，一般情况下用it作形式主语，而把动词不定式（从句）放在后面。也可以直接用不定式作主语。例如：
It is wrong to tell lies. = To tell lies is wrong. 撒谎是不对的。
但在疑问句和感叹句中，只能用it作形式主语。例如：
Isn't it a good idea to have a walk around? 出去兜一圈难道不是一个好主意吗？
What a pleasure it is to work with you! 同你一起工作是多么愉快啊！
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.