it的概念：
it可用作人稱代詞、指示代詞、先行詞及引導(dǎo)詞等。
it 的用法：
1、it可指天氣、溫度、時間、距離等 。
如：It is cold today, isn't it?
2、用作人稱代詞，代替前文提到過的事物。
如：The dog is not acold-blooded animals. It doesn't need to hibernate.
3、為避免重復(fù)，it可用來代替前面說過的短語或句子。
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、代替指示代詞this，that。
如：—What's this?
—It's an album.
—Whose new bike is that?
—It's Mary's.
注：it與one，that的區(qū)別：
it=the（this, that）+名詞，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+名詞，one指前面提到的同類事物中的不同的另一個。
如：He needs a computer, but he can't afford one.
that=the+名詞，that指代的名詞與前面的名詞屬于同一類，但不屬同一個。
如：The population of China is larger than that of Japan. that指代population，但其后有一個of短語作定語，以區(qū)別于the population of China。
注：it與that的異同：
it指同一事物，that指同類但并不是同一事物。
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 這個句型表示截止到說話時為止的某人的一種經(jīng)歷，關(guān)鍵是注意time前有序數(shù)詞，主句是一般現(xiàn)在時is時，從句要用現(xiàn)在完成時；如果主句用一般過去時was時，則從句須相應(yīng)地用過去完成時。
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、在一些相對固定的詞組中，沒有特殊含義，經(jīng)常不翻譯。
如：He's never really made it as an actor. 作為演員，他從未獲得過真正的成功。
It is my turn. 輪到我了。
強調(diào)句中的it：
可以用來改變句子結(jié)構(gòu)，使句子的某一成分得到強調(diào)：
1）強調(diào)句的基本句型it's/was+被強調(diào)成分+that/who+其他成分
原句：I told Jim the news in our office yesterday.
強調(diào)主語：It was I that/who told Jim the news in our office yesterday.
強調(diào)賓語：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.
強調(diào)地點狀語：It was in our office that I told Jim the news yesterday.
強調(diào)時間狀語：It was yesterday that I told Jim the news in our office.
2）強調(diào)句的一般疑問句型Is/Was+it+所強調(diào)部分+that/who...?
如：Was it you that told Jim the news in your office yesterday?
3）強調(diào)句的特殊疑問句型疑問詞+is/was+it+that/who...?
如：Who was it that told Jim the news in your office yesterday?
【注】強調(diào)句與主語從句雖然在形式上很相似，即都含有it is(was)...that。但，區(qū)別在于：強調(diào)句去掉it is(was)…that之后，句子結(jié)構(gòu)仍然完整，而主語從句卻不能這樣。
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。由此得出該句不是強調(diào)句，而是一個簡單的主語從句，it是形式主語，從句是真正的主語。
“it”的用法：
1、it 作形式主語：
it 在句中可作形式主語，而真正作主語的主語從句需要放在句子的末尾。主語從句后置常用以下幾種結(jié)構(gòu)：
1）It is/was+adj.+subject-clause可用于此句型的形容詞有：
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. 該句型中的形容詞通常表示事物的特點或特征的，如：
difficult, hard, easy, impossible, necessary, important等，此時用for；或表示人的性格特征或特點的，如:
nice, good, bad, kind, silly, foolish, wise, clever, careless, rude, brave, cruel, careful, grateful等，這時要用of 。
3）It is/was+名詞詞組+subject-clause可用于該結(jié)構(gòu)的名詞詞組有：
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可用于該結(jié)構(gòu)的動詞的過去分詞有：
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：本句還可改寫為：Something is said to have been done to end the pollution.
5）It+vi.+subject-clause可用于該結(jié)構(gòu)的動詞有：appear, seem, happen, occur等。
如：It appeared to scientists that the stars had moved.
6）It doesn't matter(makes no difference,etc.) +連接代詞或連接副詞引起的從句作賓語。
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（做什么是沒有用處的）
如：It's no use arguing with him.
It is/was no good doing（做什么是沒有好處的）
8）以下句型結(jié)構(gòu)中需要用虛擬語氣
�、� It is/was important(necessary, strange) that...;
It is/was ordered(required, suggested, proposed) that...;
It is/was a pity(a shame) that...表示遺憾等感情的句子中，主語從句要用“should+動詞原形”，should可以省略。
如：It's necessary that he(should) be operated on at once.
�、� It is (high)time that...結(jié)構(gòu)中用should+動詞原形（should不能省略）或動詞過去式。
如：It is high time that you should make(made) full use of your time to go over your lessons.
2、it作形式賓語：
1）動詞consider（feel, find, think等）+it+形容詞（名詞）+不定式（動詞-ing形式，從句）。
如：She thinks it no use telling me.
2）主語+appreciate（enjoy，like，love，hate）+it+if（when）...結(jié)構(gòu)
如：We would appreciate it if you could come to help us.
3）dependon, relyon, see to（負責(zé)/設(shè)法做到）, takeforgranted（習(xí)以為常）等短語后跟that從句時，要以it作形式賓語。
如：We're depending on it that he will finish the job by Friday.
“it ”引起的幾個易混淆的時間句型：
1）It be+時間+since-clause 這個句型表示從since從句謂語動作發(fā)生以后到現(xiàn)在或過去所經(jīng)過的一段時間，意為“自從…以來已多久了”，主句多用一般現(xiàn)在時，從句用一般過去時，如果表示過去的情況，主句一般用過去時，從句用過去完成時，或主句用過去完成時，從句用一般過去時。
注：since引導(dǎo)時間狀語從句時，從句若使用終止性動詞，則表示該時間是主句時間段的終點（時間從現(xiàn)在算起）；若從句使用延續(xù)性動詞，則表示該動作狀態(tài)的結(jié)束（時間從過去算起）。
如：It's five years since they got married. 他們結(jié)婚已經(jīng)5年了。
It's five years since they were married. 他們離婚已經(jīng)5年了。
It's ten years since his father was a worker. 他父親不當(dāng)工人已經(jīng)10年了。
I haven't seen him since we were boys together. 我們長大以后再沒有見過面。
2）It be+時間+before-clause 這個句型中的時間一般為表示一段時間的詞語（如：long years, months, weeks, days, hours, minutes），主句中的謂語動詞用肯定式，意為“過多長時間才…”。主句的謂語動詞是否定式時，意為“沒過多長時間就…”。主句的時態(tài)可用過去時was或?qū)頃rwillbe；用was時，before從句的動詞用一般過去時；用willbe時，before從句常用一般現(xiàn)在時。 如：It was not long before she learned those poems by heart. 她沒過多久就背會了那些詩。
It was long before the police arrived. 過了很久警察才來。
It will be hours before he makes a decision. 要過好幾個小時他才會作出決定。
It will not be hours before we meet again. 要不了幾個小時我們還會再見面的
3）It be+時間+when-clause 這個句型中，it指時間，而且表示時間的詞語前沒有介詞（時間一般是具體時間）。主句和從句中的謂語動詞在時態(tài)上是一致的，主句是willbe，when從句用一般現(xiàn)在時代替將來時。
如：It was already 8 o'clock when we got home.
It will be late afternoon when they get there.
4）It be+時間+that-clause 這個句型是個強調(diào)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.)
比較：It was 5o'clock when he started in the morning.（5o'clock前沒有介詞，這個是定語從句）
5）It be+time+that-clause 這個句型屬虛擬語氣結(jié)構(gòu)，不管主句中用的是is或was，that從句都須用動詞的過去式或should+動詞原形（但不及物動詞通常用過去式），在time之前有時可以加上high 或about 以加強語氣。
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.