it的概念：

it可用作人称代词、指示代词、先行词及引导词等。

1、人称代词
1）it的最基本用法是作代词，主要指刚提到的事物，以避免重复:
They watched the train until it disappeared in the distance.
2）也可以指动物或婴儿(未知性别的婴儿或孩子): 
Is this your dog？No, it isn’t.
They got a baby and it was a ten-pounder
3）也可指抽象事物或指抽象环境和情景:
I hate it when people talk with a full mouth.．
2、非人称代词
1）it有时并不指具体的东西而泛指天气、时间、日期、距离、价值、度量、温度、环境等： 
（1）指天气：It is a lovely day, isn’t it?
（2）指时间：It was nearly midnight when she came back.
（3）指日期：It is April First today.
（4）指距离：It is some 3000 kilometers from Beijing to Guangzhou.
（5）指价值：It is three dollars.
（6）指温度：Today it is 30 degrees centigrade.
3、其他用法
1）在句子的主语不太明确时充当主语，表示谁在做某事：
   Who is it there? It's I (me/you/he.....).
   I thought it was Mary, but it was not she.
   Her face lighted when she saw who it was.
2）泛泛的指某件事： (有时泛指一般情况)
   It doesn’t matter.
   It is a shame, isn’t it?
   How is it going? (情况怎样)
   It says in the newspaper that......
3）it用在一些词组中，it 没有特别的意思
   The last train's gone. Come on, we'll foot it. (来，咱们步行吧。)
4、作形式主语，替代主语从句，动词不定式，或动名词短语：　　　
1）作形式主语替代主语从句
  （1）It is clear ( obvious， true， possible， certain....) that 从句 常译为＂┅清楚
       的（显然的，真的，可能的,肯定的．．．）＂　　
      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.
（2）It is important ( necessary， right， strange， natural...) that 从句  常译为┅是重
     要的(必要的,对的,奇怪的,自然的┅).that 后的从句中要用虚拟语气（should + 动词原形），
     should 可以省去，建议记住该句型中的形容词。
     It is important that we (should) learn English well. 　　
     It is necessary that he (should) remember these words.
（3）It is said (reported/ learned/believed/thought/known/told/hoped.....) that 从句 常译为
    “据说（据报道，据悉…）”。 　　
     It is said that he has come to Beijing. 　　
     It is reported that another earth satellite has been put into orbit.
（4）It is suggested ( advised/ ordered / demanded/ insisted/ commanded... ) that 从句that　
     后的从句要用虚拟语气（should + 动词原形），should 可以省；常译为＂据建议；有命令．．．） 　
     It is suggested that the meeting ( should ) be put off. 　
     It was ordered that we ( should ) arrive there in two hours.
（5）It is time ( about time ，high time ) that从句(虚拟语气:动词用过去时did)
     It is time that children went to bed.
     It is time you bought a new car.
     It is (high) time you made up your mind.
（6）It is the first (second...) time that从句 (从句用现在完成时 have done ) 
     It was the first (second...) time that从句 (从句用过去完成时had done )
     常译为＂是第一（二）．．．次．．．＂。
     It is the first time I have been here. = This is the first time I have been here
（7）It is a pity ( a shame /an honour/a good thing/a fact,/a surprise/... ) that从句 
     that后的从句一般用虚拟语气（should + 动词原形），should可省去，表示出乎意料，常译
     为＂竟然＂。 没有这种意义时，则不用虚拟语气。
     It is a pity that such a thing (should) happen in your class.
     这种事竟然发生在你们班上，真是遗憾！ 　
     It is a pity that he is ill. 他生病了，真遗憾！
（8）It happens (seems，looks，appears) that从句.常译为 “ 碰巧…似乎是…,看起来…”
     It happened (so happened) that he met his teacher in the street. 碰巧…
     It seems that he will be back in a few days. 看来…
2）作形式主语替代不定式　　　
（1）It is kind ( of sb. ) to do sth. 不定式的逻辑主语是由of引起，主句中的形容词必须
     是能表示逻辑主语特征的褒义或贬义形容词。 常见的词有：
     bad，brave，careless，clever，cruel，foolish，good（好心的），honest，horrible，kind，
     lazy，modest， naughty，nice（有教养的），polite，rude，silly，stupid，wise，wrong
     （错误的）等。
这个句型可以改写为：sb. is kind to do sth. 。　
如：It is kind of you to say so. = You are kind to say so.
（2）It is necessary ( for sb. ) to do sth. 不定式的逻辑主语是由for引起，主句中的形容
     词通常是表示重要性，紧迫性，频繁程度，难易，安全等情况的中性形容词。 常见的形容词有： 
     important，necessary，natural easy，safe，common，normal，hard，difficult，dangerous，
     unusual，rare， impossible，pleasant
     如：It is important for her to come to the party. = It is important that she (should)
         come to the party.
（3）It takes sb. ... to do sth. 常译为“做…要花费某人…”。 　　
     如：It took thousands of people many years to build the Great Wall.
3）作形式主语替代动名词短语
   It is no good / no use / useless doing sth. 常译为 “┅有好处或没有用” 　　
   It is no good learning English without speaking English.
   It's useless trying to argue with Shylock.
5、作形式宾语，代替不定式，动名词短语或宾语从句。
   We think it important to learn a foreign language.
该句型中的it 作形式宾语，该结构中常用的动词有：think, believe, make ,find consider,
feel； 如:
We think it our duty to clean our classroom every day. 　
He felt it important learning English well. 　
They found it difficult that they would finish their work in two days.
The Internet makes it easier for companies to keep in touch with customers.
6、it的重要句型
1）强调句型:  It is/was  + 被强调部分 + that 从句 (被强调的主语如果是人，that可以由who换用)
It was about 600 years ago that the first clock with a face and an hour hand was made.
It was they that (who ) cleaned the classroom yesterday.
It was in the street that I met her father.
It was yesterday that I met her in the street.
It is you that /who are wrong.
特例：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.

it 的用法：

1、it可指天气、温度、时间、距离等 。
如：It is cold today, isn't it?
2、用作人称代词，代替前文提到过的事物。
如：The dog is not  acold-blooded animals.  It doesn't need to hibernate.
3、为避免重复，it可用来代替前面说过的短语或句子。
如：I tried to persuade my father to give up smoking, but found it impossible. (it=to persuade my father to give up smoking)
4、代替指示代词this，that。
如：—What's this?
        —It's an album.
        —Whose new bike is that?
        —It's Mary's.
注：it与one，that的区别：
        it=the（this, that）+名词，特指并且代替前面所提到的某特定事物。
如：He's bought a new car, so he drives it everywhere to show ito ff.
        one=a+名词，one指前面提到的同类事物中的不同的另一个。
如：He needs a computer, but he can't afford one.
        that=the+名词，that指代的名词与前面的名词属于同一类，但不属同一个。
如：The population of China is larger than that of Japan.  that指代population，但其后有一个of短语作定语，以区别于the population of China。
注：it与that的异同：
        it指同一事物，that指同类但并不是同一事物。
如：I like the climate of Kunming more than that of Beijing.
       The climate of Kunming is mild, and I like it.
5、It/This/That+be+the first(second, third...) time+that-clause 这个句型表示截止到说话时为止的某人的一种经历，关键是注意time前有序数词，主句是一般现在时is时，从句要用现在完成时；如果主句用一般过去时was时，则从句须相应地用过去完成时。
如：This is the first time(that) these Europeans have visited the Great Wall.
        It was the fifth time(that) I had paid a friendly visit to America.
6、在一些相对固定的词组中，没有特殊含义，经常不翻译。
如：He's never really made it as an actor. 作为演员，他从未获得过真正的成功。
        It is my turn. 轮到我了。

强调句中的it：

可以用来改变句子结构，使句子的某一成分得到强调：
1）强调句的基本句型it's/was+被强调成分+that/who+其他成分
原句：I told Jim the news in our office yesterday.
强调主语：It was I that/who told Jim the news in our office yesterday.
强调宾语：It was Jim that I told the news in our office yesterday.
            或：It was the news that I told Jim in our office yesterday.
强调地点状语：It was in our office that I told Jim the news yesterday.
强调时间状语：It was yesterday that I told Jim the news in our office.
2）强调句的一般疑问句型Is/Was+it+所强调部分+that/who...?
如：Was it you that told Jim the news in your office yesterday?
3）强调句的特殊疑问句型疑问词+is/was+it+that/who...?
如：Who was it that told Jim the news in your office yesterday?
【注】强调句与主语从句虽然在形式上很相似，即都含有it is(was)...that。但，区别在于：强调句去掉it is(was)…that之后，句子结构仍然完整，而主语从句却不能这样。
如：(It is)our hope(that) the two sides will work towards peace.
解析：去掉It is…that之后，句子是不成立的。由此得出该句不是强调句，而是一个简单的主语从句，it是形式主语，从句是真正的主语。

“it”的用法：

1、it 作形式主语：
it 在句中可作形式主语，而真正作主语的主语从句需要放在句子的末尾。主语从句后置常用以下几种结构：
1）It is/was+adj.+subject-clause可用于此句型的形容词有：
clear, certain, funny, good, impossible, likely, natural, obvious, possible, probable, strange, surprising, true, unusual, wonderful等。
如：It is obvious that going for sports will do a lot of good to your health.
2）It+be+adj./n.(forsb./ofsb.)+todosth. 该句型中的形容词通常表示事物的特点或特征的，如：
difficult, hard, easy, impossible, necessary, important等，此时用for；或表示人的性格特征或特点的，如:
nice, good, bad, kind, silly, foolish, wise, clever, careless, rude, brave, cruel, careful, grateful等，这时要用of 。
3）It is/was+名词词组+subject-clause可用于该结构的名词词组有：
a pity/duty, a good thing, no surprise, good news, an honor, a fact, a mystery, a shame, manners等。
 如：It's a pity that I didn't attend the party.
4）It is/was+V-ed+subject-clause可用于该结构的动词的过去分词有：
said, reported, thought, supposed, believed, hoped, expected, known, decided, announced, arranged等。
如：It is said that something had been done to end the pollution.
注：本句还可改写为：Something is said to have been done to end the pollution.
5）It+vi.+subject-clause可用于该结构的动词有：appear, seem, happen, occur等。
 如：It appeared to scientists that the stars had moved.
6）It doesn't matter(makes no difference,etc.) +连接代词或连接副词引起的从句作宾语。
如：It doesn't matter whether he'll join the army or not.
        It makes no difference where we have the conference.
7）一些固定句型：
It takes sb. some time to do sth.
如：It will take you two days to get there on foot. 
It costs sb. some money to do sth.
如：It costs 1,000 dollars to fly to America.
It is/was no use(useless) doing（做什么是没有用处的）
如：It's no use arguing with him.
It is/was no good doing（做什么是没有好处的）
8）以下句型结构中需要用虚拟语气
① It is/was important(necessary, strange) that...;
     It is/was ordered(required, suggested, proposed) that...;
     It is/was a pity(a shame) that...表示遗憾等感情的句子中，主语从句要用“should+动词原形”，should可以省略。
如：It's necessary that he(should) be operated on at once.
② It is (high)time that...结构中用should+动词原形（should不能省略）或动词过去式。
如：It is high time that you should make(made) full use of your time to go over your lessons.

2、it作形式宾语：
1）动词consider（feel, find, think等）+it+形容词（名词）+不定式（动词-ing形式，从句）。
 如：She thinks it no use telling me.
2）主语+appreciate（enjoy，like，love，hate）+it+if（when）...结构
如：We would appreciate it if you could come to help us.
3）dependon, relyon, see to（负责/设法做到）, takeforgranted（习以为常）等短语后跟that从句时，要以it作形式宾语。
如：We're depending on it that he will finish the job by Friday.

“it ”引起的几个易混淆的时间句型：

1）It be+时间+since-clause 这个句型表示从since从句谓语动作发生以后到现在或过去所经过的一段时间，意为“自从…以来已多久了”，主句多用一般现在时，从句用一般过去时，如果表示过去的情况，主句一般用过去时，从句用过去完成时，或主句用过去完成时，从句用一般过去时。
注：since引导时间状语从句时，从句若使用终止性动词，则表示该时间是主句时间段的终点（时间从现在算起）；若从句使用延续性动词，则表示该动作状态的结束（时间从过去算起）。
如：It's five years since they got married. 他们结婚已经5年了。 
        It's five years since they were married. 他们离婚已经5年了。
        It's ten years since his father was a worker. 他父亲不当工人已经10年了。
        I haven't seen him since we were boys together. 我们长大以后再没有见过面。 
2）It be+时间+before-clause 这个句型中的时间一般为表示一段时间的词语（如：long years, months, weeks, days, hours, minutes），主句中的谓语动词用肯定式，意为“过多长时间才…”。主句的谓语动词是否定式时，意为“没过多长时间就…”。主句的时态可用过去时was或将来时willbe；用was时，before从句的动词用一般过去时；用willbe时，before从句常用一般现在时。 如：It was not long before she learned those poems by heart. 她没过多久就背会了那些诗。
        It was long before the police arrived. 过了很久警察才来。
        It will be hours before he makes a decision. 要过好几个小时他才会作出决定。
        It will not be hours before we meet again. 要不了几个小时我们还会再见面的
3）It be+时间+when-clause 这个句型中，it指时间，而且表示时间的词语前没有介词（时间一般是具体时间）。主句和从句中的谓语动词在时态上是一致的，主句是willbe，when从句用一般现在时代替将来时。
如：It was already 8 o'clock when we got home.
        It will be late afternoon when they get there. 
4）It be+时间+that-clause 这个句型是个强调句型。
如：It was at 5o'clock that he practiced playing the violin in the morning. (原句是：He practiced playing the violin at 5o'clock in the morning.)
比较：It was 5o'clock when he started in the morning.（5o'clock前没有介词，这个是定语从句）
5）It be+time+that-clause 这个句型属虚拟语气结构，不管主句中用的是is或was，that从句都须用动词的过去式或should+动词原形（但不及物动词通常用过去式），在time之前有时可以加上high 或about 以加强语气。
如：It is high time(that) he wrote a letter to his girl friend.
        It is time(that) we made people's life a little easier.= It is time that we should make people's life a little easier.

1、掌握it的指代内容如时间、距离、度量衡等。
2、掌握it的重点句型。