引導(dǎo)詞it的用法
（1）作形式主語
代替由不定式（或不定式復(fù)合結(jié)構(gòu)）、動名詞或從句所表示的真正主語.
eg:It was wrong for you not to help her.
你當(dāng)時不幫助她是錯誤的.(it代替不定式短語)
It will be useless learning a theory without practice.
學(xué)習(xí)理論而沒有實(shí)踐是無用的.(it 代替動名詞短語)
It’s not yet known where she has gone.
她去哪里了還不知道.(it代替主語從句)
（2）作形式賓語
代替由不定式、動名詞或從句所表示的真正賓語.eg:
We consider it necessary to master two foreign languages.
我們認(rèn)為掌握兩門外語是有必要的.(it作動詞不定式的先行詞)
We found it useless talking to her.
我們認(rèn)為跟她談是無用的.(it作動名詞的先行詞)
I must make it clear that I’ll always support you.
我必須申明我將永遠(yuǎn)支持你.(it作that引出的賓語從句的先行詞)
（3）用在強(qiáng)調(diào)結(jié)構(gòu)中
為了強(qiáng)調(diào)句子的某一成分（通常是主語、賓語或狀語）,常用“It is(was) +被強(qiáng)調(diào)成分+that（或who）……”結(jié)構(gòu).eg:
I met Tom in the street yesterday.我昨天在街上遇見了湯姆.
It was I who met Tom in the street yesterday..是我昨天在街上遇見湯姆的.
It was Tom that I met in the street yesterday.昨天我在街上遇見的是湯姆.
It was in the street that I met Tom yesterday.我昨天是在街上遇見湯姆的.
It was yesterday that I met Tom in the street.我是昨天在街上遇見湯姆的.
注：如果去掉“It is (was) ……”這一結(jié)構(gòu),將被強(qiáng)調(diào)部分放回原句,其句子仍然成立.eg:
Where is it that we shall park our car?我們把車停在哪?
（去掉is it that,原句還原成:Where shall we park our car?）
3.幾種易混句型：
(1) ①It was four years since I had left the school.我畢業(yè)4年了.
②It was four years that the war lasted.戰(zhàn)爭持續(xù)了4年.
注：①since引導(dǎo)時間狀語,不能用that替代；
②強(qiáng)調(diào)句型,強(qiáng)調(diào)了時間狀語.
(2) ①Was it 1949 when our town was liberated?
我們鎮(zhèn)解放的時間是1949年嗎?
②Was it in 1949 that our town was liberated?
我們鎮(zhèn)是在1949年解放的嗎?
注：①It指時間,when在從句中作狀語,不能由that充當(dāng)；
②強(qiáng)調(diào)句型.
(3) ①It is (about,high) time that you closed your store.
到你關(guān)閉商店的時候了.
②It is the first time that I have been to the Great Wall.
這是我第一次去長城.
注：①虛擬語氣句型,從句謂語動詞要用過去式表示對現(xiàn)在情況的虛擬；
②that習(xí)慣用法,不能用when代替.
(4) ①It is a pity that Tom has lost the game.很遺憾,湯姆比賽失敗了.
②It is Tom that has lost the game.是湯姆比賽失敗了.
注：①It作形式主語；②It引導(dǎo)詞,強(qiáng)調(diào)句型.
(查了好久的)