如题所述
1、it的基本用法
（1）用作人称代词，代替前文提到过的事物。
The train has arrived. It arrived half an hour ago.
（2）用以代替提示代词this, that。
—What’s this? —It’s a knife.
—Whose watch is that? —It’s mine.
（3）起指示代词的作用，指一个人或事物。
—Who is knocking at the door? —It’s me.
（4）指环境情况等。
It was very noisy (quiet) at the very moment.
（5）指时间、季节等。
—What time is it? —It’s eight o’clock.
It often rains in summer here.
（6）指距离。
It is a long way to the school.
（7）作形式主语。
It is not easy to finish the work in two days.
It is no use crying over spilt milk.
It is as pity that you didn’t read the book.
（8）作形式宾语。
I think it no use arguing with him.
I found it very interesting to study English.
He made it clear that he was not interested in that subject.
（9）用于强调结构。
It was Xiaoming whom(that)I met in the street last night.
It was in the street that I met Xiaoming last night.
It ws last night that I met Xiaoming in the street.
It was I who met Xiaoming in the street last night.
2、含有“It is …”的句型
（1）It is time (for sb.) to do sth.
It is (high) time that sb. did sth.(虚拟语气)
（2）It is + 形容词（+of / for sb.）+ to do sth.
通常用
of的词有brave、clever、careful、hopeless、kind、good、naughty、nice、silly、stupid、foolish、wise等。
（3）It is + 形容词 + that + sb. + (should) do sth.（虚拟语气）
能用于这个句型的形容词有strange、wonderful、natural、good、proper、right、wrong、well、fortunate、important、necessary、useless、likely、probable、impossible等。有时可省去should而直接用动词原形。如：
It is important that we should pay close attention to grain.
It is natural that he（should）say so.
（4）It is no use/ good + doing sth.
It is no use trying again = It is of no use to try again.
（5）It is + 被强调部分 + that/who + …
Was it in the street that you met her?
Who was it that called him“comrade”?
It was not until yesterday that I met my old friend.
It is in a small factory that my brother is going to
work.（注意上述各句均为强调句型的“考点”）
比较：It is a small factory where my brother is going to
work.（定语从句）
（6）It is said/reported/announced/ (well) known … that…
（7）It is/was + 时间 + since … 从……已多久了。
It is three years since I met him in Beijing.
It was a long time since I (had) lived in that small mountain
village with these farmers.（注意两句中的时态）
It is five months since I arrived in New York.我到纽约已经五个月了。
It is five months since I was in New York.我离开纽约已经五个月了。
（注意：终止性动词表示从动作发生时算起；如果是状态动词或持续性动词，则从动作或状态结束时算起。）
（8）It is + 时间 + before …
这个句型和上面句型中时间都是时间段，如long、years、months、weeks、five days、three
hours、twenty minutes等。不过主句的时态多用一般将来时或一般过去时。
It will be several years before we meet again.
我们要过好几年才能再见面。
It was not long before they set out for the front.
不久他们就出发去了前线。
（9）It is /was/will be the first/second/third … time that …
It is the first time that I have been here.
It was the second time that he had seen the film.
（10）It is up to sb. to do sth … 应由某人做某事。
It is up to you to decide whether we start or not.
（1）用作人称代词，代替前文提到过的事物。
The train has arrived. It arrived half an hour ago.
（2）用以代替提示代词this, that。
—What’s this? —It’s a knife.
—Whose watch is that? —It’s mine.
（3）起指示代词的作用，指一个人或事物。
—Who is knocking at the door? —It’s me.
（4）指环境情况等。
It was very noisy (quiet) at the very moment.
（5）指时间、季节等。
—What time is it? —It’s eight o’clock.
It often rains in summer here.
（6）指距离。
It is a long way to the school.
（7）作形式主语。
It is not easy to finish the work in two days.
It is no use crying over spilt milk.
It is as pity that you didn’t read the book.
（8）作形式宾语。
I think it no use arguing with him.
I found it very interesting to study English.
He made it clear that he was not interested in that subject.
（9）用于强调结构。
It was Xiaoming whom(that)I met in the street last night.
It was in the street that I met Xiaoming last night.
It ws last night that I met Xiaoming in the street.
It was I who met Xiaoming in the street last night.
2、含有“It is …”的句型
（1）It is time (for sb.) to do sth.
It is (high) time that sb. did sth.(虚拟语气)
（2）It is + 形容词（+of / for sb.）+ to do sth.
通常用
of的词有brave、clever、careful、hopeless、kind、good、naughty、nice、silly、stupid、foolish、wise等。
（3）It is + 形容词 + that + sb. + (should) do sth.（虚拟语气）
能用于这个句型的形容词有strange、wonderful、natural、good、proper、right、wrong、well、fortunate、important、necessary、useless、likely、probable、impossible等。有时可省去should而直接用动词原形。如：
It is important that we should pay close attention to grain.
It is natural that he（should）say so.
（4）It is no use/ good + doing sth.
It is no use trying again = It is of no use to try again.
（5）It is + 被强调部分 + that/who + …
Was it in the street that you met her?
Who was it that called him“comrade”?
It was not until yesterday that I met my old friend.
It is in a small factory that my brother is going to
work.（注意上述各句均为强调句型的“考点”）
比较：It is a small factory where my brother is going to
work.（定语从句）
（6）It is said/reported/announced/ (well) known … that…
（7）It is/was + 时间 + since … 从……已多久了。
It is three years since I met him in Beijing.
It was a long time since I (had) lived in that small mountain
village with these farmers.（注意两句中的时态）
It is five months since I arrived in New York.我到纽约已经五个月了。
It is five months since I was in New York.我离开纽约已经五个月了。
（注意：终止性动词表示从动作发生时算起；如果是状态动词或持续性动词，则从动作或状态结束时算起。）
（8）It is + 时间 + before …
这个句型和上面句型中时间都是时间段，如long、years、months、weeks、five days、three
hours、twenty minutes等。不过主句的时态多用一般将来时或一般过去时。
It will be several years before we meet again.
我们要过好几年才能再见面。
It was not long before they set out for the front.
不久他们就出发去了前线。
（9）It is /was/will be the first/second/third … time that …
It is the first time that I have been here.
It was the second time that he had seen the film.
（10）It is up to sb. to do sth … 应由某人做某事。
It is up to you to decide whether we start or not.
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第1个回答 2008-02-20
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第2个回答 2008-02-20
it的用法
1、以it作主语的句子
1）代替刚提到的一件事物
What’s this? It’s a book.
I knew nothing about it until you told me.
2）起指示代词的作用，指一个人或事物
Who is knocking at the door? It’s me.
3）表示时间、天气、距离等
It is late autumn now.
It’s rather windy today.
It’s only five miles to the Summer Palace.
It was very quiet at the moment.
4）代替一个由不定式、动名词短语或是从句的主语，使原来的这些主语可以放在句子的后部，免得句子头重脚轻。
（1）用不定式作主语的这类句子很多。
It’s our duty to do it.
It is necessary for us to have some exercise every day.
（2）能以动名词作主语的这类句子比较少，主要在以no use, useless, dangerous, waste, worthwhile等词作表语的句子中用。
It’s no use talking to him about it.
It’s dangerous playing with fire.
Is it worthwhile making such an experiment?
It is a waste of time your talkiing to him.
（3）用从句作主语的这类句子是不少的，这个从句可以用连词that 引起，也可以用一个连接代词或连接副词引起。
It is very likely that their group will get ahead of us.
It was clear that they had no desire for peace.
It seems that Mr Green’s idea is more practical.
It is doubtful whether she will be able to come.
It was clear enough what she meant.
It is said that he’s doing fine at school.
It doesn’t matter what you do.
2、以it作宾语的句子
当复合宾语中的宾语是动词不定式、动名词、宾语从句时，往往把宾语放在它的补足语的后面，而用引词it放在谓语动词和宾语补足语中间。it叫做形式宾语。
1)不定式
She found it very difficult to answer the question.
He feels it his duty to help others.
He made it a rule to talk to him in English every day.
2)动名词
I think it no use telling them.
You must find it exciting working here.
3)宾语从句
We have made it clear that learning English is important.
I think it very important that we should take part in the discussion.
They kept it quiet that he was dead.
三、it 用于强调结构
改变一个句子的结构，强调这个句子中的某一成分。
强调句式结构：It +is/was + 要强调部分+that (who) +句子其他部分。
几点说明：1、被强调的确部分是人时，既可用who 也可用that；2、强调的成分在句中作宾语可用whom，也可省略；3、强调时间和地点决不用when 和where，一律用that。
以Jim met your sister in the zoo yesterday.为例，借助强调句式可改为下面几个形式，各强调一个不同的成分。
It was Jim who (或that) met your sister in the zoo yesterday.（强调主语）
It was your sister that Jim met in the zoo yesterday. （强调宾语）
It was in the zoo that (不用where) Jim met your sister yesterday.（强调地点状语）
It was yesterday that (不用when) Jim met your sister in the zoo.（强调时间状语）
强调结构也可用于强调短语和从句。
It was because I wanted to buy a dictionary that I went to town yesterday.
1、以it作主语的句子
1）代替刚提到的一件事物
What’s this? It’s a book.
I knew nothing about it until you told me.
2）起指示代词的作用，指一个人或事物
Who is knocking at the door? It’s me.
3）表示时间、天气、距离等
It is late autumn now.
It’s rather windy today.
It’s only five miles to the Summer Palace.
It was very quiet at the moment.
4）代替一个由不定式、动名词短语或是从句的主语，使原来的这些主语可以放在句子的后部，免得句子头重脚轻。
（1）用不定式作主语的这类句子很多。
It’s our duty to do it.
It is necessary for us to have some exercise every day.
（2）能以动名词作主语的这类句子比较少，主要在以no use, useless, dangerous, waste, worthwhile等词作表语的句子中用。
It’s no use talking to him about it.
It’s dangerous playing with fire.
Is it worthwhile making such an experiment?
It is a waste of time your talkiing to him.
（3）用从句作主语的这类句子是不少的，这个从句可以用连词that 引起，也可以用一个连接代词或连接副词引起。
It is very likely that their group will get ahead of us.
It was clear that they had no desire for peace.
It seems that Mr Green’s idea is more practical.
It is doubtful whether she will be able to come.
It was clear enough what she meant.
It is said that he’s doing fine at school.
It doesn’t matter what you do.
2、以it作宾语的句子
当复合宾语中的宾语是动词不定式、动名词、宾语从句时，往往把宾语放在它的补足语的后面，而用引词it放在谓语动词和宾语补足语中间。it叫做形式宾语。
1)不定式
She found it very difficult to answer the question.
He feels it his duty to help others.
He made it a rule to talk to him in English every day.
2)动名词
I think it no use telling them.
You must find it exciting working here.
3)宾语从句
We have made it clear that learning English is important.
I think it very important that we should take part in the discussion.
They kept it quiet that he was dead.
三、it 用于强调结构
改变一个句子的结构，强调这个句子中的某一成分。
强调句式结构：It +is/was + 要强调部分+that (who) +句子其他部分。
几点说明：1、被强调的确部分是人时，既可用who 也可用that；2、强调的成分在句中作宾语可用whom，也可省略；3、强调时间和地点决不用when 和where，一律用that。
以Jim met your sister in the zoo yesterday.为例，借助强调句式可改为下面几个形式，各强调一个不同的成分。
It was Jim who (或that) met your sister in the zoo yesterday.（强调主语）
It was your sister that Jim met in the zoo yesterday. （强调宾语）
It was in the zoo that (不用where) Jim met your sister yesterday.（强调地点状语）
It was yesterday that (不用when) Jim met your sister in the zoo.（强调时间状语）
强调结构也可用于强调短语和从句。
It was because I wanted to buy a dictionary that I went to town yesterday.
参考资料：
