高中英语it的用法it的概念：it可用作人称代词、指示代词、先行词及引导词等。
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①It 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.②It 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。