It 作形式主语的四种结构
It作形式主语
It作形式主语和形式宾语，虽然是我们高中就学过的语法点，但在考研英语题型中，无论是在阅读还是翻译，常会考查该知识点。
一.it作形式主语。
英语中不定式、动名词、主语从句在作主语时主语往往显得很长，使整个句子看上去有点头重脚轻，这样就借助“it”来作形式上的主语，而把真正的主语移到句子后边去，使全句结构上前后平衡。
“it”并无实际意义。
It之后的谓语部分，可以是be+形容词，be+名词(词组)，或be+过去分词等。
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：一、不定式作真正主语主要用于下列句型：1. It + is/was + adj./n.+ to do sth.A.名词作表语。
主要有a pity, a pleasure, an idea等。
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。
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。
2. 其他形容词作表语，常见的形容词有：necessary, hard，important, difficult, easy, possible，common等。
it作为形式主语
形式主语形式主语是动词不定式的一种,它也是逻辑主语, 可以把它归类到逻辑主语的第一种(不定式的逻辑主语) 用作宾语的不定式，用作目的状语，用作结果状语的不定式都是属于不定式的大概念里面的.形式主语： It +be+(the)+adj+for sb+to do sth.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：1. 句子的逻辑主语为不定式如：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.2. 句子的逻辑主语为从句如：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 hiddenbetween the layers of the briefcase.3. 句子的逻辑主语为动名词短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。
“it”引导主语从句和“it”做形式主语还有强调句型的区别分别是什么？
“it”引导主语从句和“it”做形式主语还有强调句型的区别：it引导的主语从句，形式主语就是it，真正的主语在句子的后面，通常是to do的不定式短语。
而强调句的结构
it is/was+被强调部分+that/who+句子其余部分，结构词it is/was和that/ who都是没有实际意思的，只是构成句子结构的词而已。
这个强调句只能强调名词和代词，也就是主语和宾语，不能强调其他句子成分。
主语从句：It is kind of you to help me with my housework.
强调句：It is you who help me with my housework.
主语从句中it做形式主语的常见句型：It+be+形容词+主语从句，如：It is u ncertain whether he can come to Jenny’s birthday party or not. It + be + 名词词组+ 主语从句，这类名词有：a fact, a pity, a shame, an honour，a question等，如：
It’s a pity that you missed the exciting football match.
It remains a question whether he will come or not.
It + be + 过去分词+ 主语从句，这类过去分词有：announced, believed, expected, hoped, decided, reported, said, shown等，如：It is reporte that 16 people were killed in the earthquake.。
it作形式主语和宾语的几种结构
“It”作形式主语和宾语It作形式主语和形式宾语用法，是中学阶段英语学习的主要语法项目之一。
纵观历届高考题，无论是单项选择，还是完形填空，it用法始终是反复考查的重、热点之一。
现将it在特殊句型中作形式主语和形式宾语用法，进行如下归纳分析。
一、It 用作形式主语当不定式（短语）、动名词（短语）或从句在某个句子中作主语时，为保持句子结构前后平衡，避免头重脚轻，因此常用it作形式主语置于句首，而将真正的主语放在句尾。
此时it只起先行引导作用，本身无词义。
e.g. It is wrong to tell a lie.(说谎是错误的。
) 〔It为to tell a lie的形式主语〕It is no use arguing about it.(争吵是没用的。
) 〔It为arguing about it的形式主语〕It is uncertain who will come.(谁要来还不确定。
) 〔It为who will come的形式主语〕It 作形式主语的常见句型：① It + be + 形容词+ to do sth. / doing / that ….e.g. It is very important to learn a foreign language.(学一门外语非常重要。
)It is useless crying over the spilt milk.(覆水难收。
)It was really surprising that she married a man like that.(她嫁给那样的男士真让人惊讶。
)② It + be +名词词组+ doing / that ….e.g. It is no good telling lies.(撒谎没好处。
)It is a pity that you didn’t go to see the film yesterday.(你昨天没看成那部电影真遗憾。
it做形式主语用法大全含练习及答案
It作形式主语常见句型动词不定式，动名词或主语从句作主语时，通常把它们放在谓语动词前，而用it作形式主语，放到句首，常见句型有：1. It + be + 形容词+主语从句，如：It is uncertain whether he can come to Jenny’s birthday party or not.2. It + be + 名词词组+ 主语从句，这类名词有：a fact, a pity, a shame, an honour，a question等，如：It’s a pity that you missed the exciting football match.It remains a question whether he will come or not.3. It + be + 过去分词+ 主语从句，这类过去分词有：announced, believed, expected, hoped, decided, reported, said, shown等，如：It is reported that 16 people were killed in the earthquake.It is not decided who will give the operation to the patient.4. It + 不及物动词(seem, appear, happen等) + 主语从句, 这类不及物动词有：appear, happen, seem等，如：It seemed that he didn’t tell the truth.It happened that I was out when he called.5. It + be + 形容词+ for sb.）+ 动词不定式这类句型常用形容词easy, difficult, hard, important, possible，wise等作表语，有时候为了强调不定式动作的执行者，常在不定式前加for sb，如：It’s necessary for the young to master two foreign language.It is unwise to give the children whatever they want.这类形容词有：easy, difficult, hard, important, possible，wise6. It + be + 形容词+ of sb. + 动词不定式，这类形容词常是表示心理品质,性格特征的形容词，如：kind, nice stupid, clever, foolish, polite, impolite, silly, selfish, considerate等。
it做形式主语的19个句型
1.It is/was + 被强调部分 + that/who ...之巴公井开创作该句型是强调句型。
被强调部分可以是主语，宾语，表语或状语。
强调的主语如果是人，that 可以由 who 换用。
如果把这种句型结构划掉后，应该是一个完整无缺的句子。
这也是判断强调句型与其它从句的方法。
2. It is not until + 被强调部分 + that ...该句型也是强调句型。
主要用于强凋时间状语，译成汉语“直到……才……”，可以说是 not ... until ... 的强调形式。
It was not until she took off her dark glasses that I realized she was a famous film star.= Not until she took off her dark glasses did I realize she was a famous film star.= I didn't realize she was a famous film star until she took off her dark glasses.3. It is clear /obvious/true/possible/cer tain… that …该句型中it 是形式主语，真正的主语是 that 引导的主语从句，常译为“清楚（显然，真的，肯定……）”是主语从句最罕见的一种结构。
It is very clear that he's round and tall like a tree. = That he's round and tall like a tree is very clear.4. It is important /necessary/right/strange/natural... that ...+(should) do该句型和上一个同属一个句型。
it作形式主语的典型句型
fast. 你
是很 的。
4. It be + 名词（形容词）+ doing sth.
it代替真正的主语----- 动名词（短 语 ），可
用于这类句型的名词或形容词有dangerous,
nice, foolish, worth, waste, no good, not any
good, no use,not any use等。例 如 ：
make frie n d s .失去朋友比交朋友更容易。
2. It be + 形 容 词 + of sb. + 动词不定式
(短语）
it代 替 真 正 的 主 语 — 动 词 不 定 式 （短
语)。动词不定式的逻辑主语是由介词of引起
的 ，形容词必须是能表示逻辑主语性格特点
的词。常见的有bad, brave,careless,clever,
unusual, rare,impossible, pleasant等。例 如 ：
① It is important for her to come to
the party. 能
很重 。
" I t is difficult for him to do the math
problem.
是 的。
③ It’s very dangerous for you to run so
。
" I t is wrong of me to pick up your
umbrella.
你的 , 是 的不 。
常用it作形式主语的句型结构
常用it作形式主语的句型结构◇It + be +形容词+ that-从句可用于此句型的形容词有: wonderful,true，good，right，wrong，（un）important，useless，surprising，clear，unusual，lucky，certain, necessary等。
如：①It is quite certain that he will be at the meeting.②It is surprising that he came late to school this morning.③It is important that we(should)study hard注意: 该句型中的形容词是（un）important, necessary等时，从句应为(should )+动词原形。
◇It + be + -ed 分词+ that-从句适用该句型的过去分词主要有: said,reported,announced,hoped,thought，told，believed，expected，decided，suggested,known等。
如：①It is hoped that one day they will have enough animals to set them free②It is well known that the earth goes around the sun.③It is suggested that the sports meeting (should ) be put off until next week. 注意: 该句型的过去分词是suggested时，从句应为(should )+动词原形。
◇It + be +名词+ that-从句适用该句型的名词（词组）有: a pity，an honor，a good thing，a fact，a surprise，good news, one’s duty等。