It 既是 代詞又是引詞。作代詞時，它可做人稱代詞、指示代詞、非人稱代詞，用于前指、非確指或習(xí)語中。

it作引詞時，它本身無實義，只起先行引導(dǎo)的作用?？勺?形式主語（it is + adj. + for/of sb. to do sth.）或 形式賓語（it + adj. + to do sth.），真實的主語或賓語是 不定式、 動名詞或名詞 從句，它們則放在后面。It 也用于 強(qiáng)調(diào)句結(jié)構(gòu)。

如想強(qiáng)調(diào)某個詞或部分，可用it is (was) + 強(qiáng)調(diào)部分（主語、賓語或狀語） + that(who）…的強(qiáng)調(diào)結(jié)構(gòu)。本章要求了解代詞it 和引詞it 的各種用法，重點掌握it 用于前指或后指；引導(dǎo)詞it 用于強(qiáng)調(diào)結(jié)構(gòu)。

例：It is Tom’s sister who have had won the first place in the 100 meter race.

注意：

有時為避免句子頭重腳輕，常用形式主語it代替 主語從句作形式主語放于句首，而把主語從句置于句末。主語從句后的 謂語動詞一般用單數(shù)形式。常用句型如下：

It is necessary (important, natural, strange, etc.) that …

It is suggested (requested, proposed, desired, etc.) that…

That- 從句做主語通常用it作先行詞，而將that-從句置于句末，例如：

It is quite clear that the whole project is doomed to failure. 很清楚，整個計劃注定要失敗。

It's a pity that you should have to leave. 你非走不可真是件憾事。

價值300元外教英語課程領(lǐng)取：http://www.krbn.com.cn/lps/lp4.htm?search=700053?(北美原版教材）
口語英語速成班：http://www.krbn.com.cn/lps/lp-tutor/mix-tutor.htm?search=700053（劍橋，新概念等教材）

用it作 形式主語的that-從句有以下四種不同的搭配關(guān)系：

a. It + be +形容詞+ that-從句

It is necessary that…　有必要……?? /?? It is important that…　重要的是……

It is obvious that…　很明顯……

例：It is significant for persons to learn English well !

b. It + be + -ed 分詞+ that-從句

It is believed that…　人們相信……

It is believed that to get a high education degree that can lead to a better salary . 人們相信獲得更高的學(xué)歷水平可以得到跟更高的薪水。

It is known to all that…　眾所周知……

It is known to all that the Beijing is the Chinese capital .?? 眾所周知北京是中國的首都

It has been decided that…　已決定……

It has been decided that Sunny will hold the meeting and give a speech .已經(jīng)決定讓Sunny主持會議并發(fā)表演講。

c. It + be +名詞+ that-從句

It is common knowledge that…　……是常識 /? It is a surprise that…　令人驚奇的是……

It is a fact that…　事實是……

It is a fact that the Taiwan Island belongs to China form thousands years ago.

d. It +不及物動詞+ that-從句

It appears that…　似乎……?? /??? It happens that…　碰巧……

It occurred to me that…　我突然想起……

It occurred to me that I have left my purse behind at home .

It's for sb.和 It's of sb.

1）? for sb. 常用于表示事物的特征特點，表示客觀形式的形容詞，如easy, hard, difficult, interesting, impossible等：

It's very hard for him to study two languages. 對他來說學(xué)兩門外語是很難的。

2）of sb的句型一般用表示人物的性格，品德，表示主觀感情或態(tài)度的形容詞，如good, kind, nice, clever, foolish, right。

It's very nice of you to help me. 你來幫助我，你真是太好了。

It作為引導(dǎo)詞，引導(dǎo)長句時候非常普遍，并且對于不會運用其他幾個復(fù)雜的從句的英語學(xué)員來說，是展示語法功底的優(yōu)勢手段。基本句型按照套路即可全部掌握。