it的用法
一、概述
在英语中，it的使用相当广泛，它既可用作代词，如人称代词（personal it）、指示代词（demonstrative it）及非人称代词（impersonal it）, 也可用作引导词（anticipatory it）和强调结构中的强调词（emphatic it）
Someone is ringing the doorbell. Go and see who it is.有人在按门铃。去看看是谁。（人称代词）
What’s this?这是什么？
It’s a book.这是一本书。（指示代词）
What a long way it is from Beijing to London! 从北京到伦敦真远。（非人称代词）
It's best to plant trees in spring because it's warmer.春天是植树的最佳时节，因为天气更暖和。（作引导词）
It was I who met him in the park last week. 是我上星期在公园遇到他的。（强调结构中的强调词）
二、it作代词
1、用作人称代词（personal it）
代替前文提到过的事物，it作真实主语或宾语。
The frog is not a warm-blooded animal. It is a cold-blooded one.青蛙不是温血动物，它是冷血动物。
My pen is missing. I can't find it anywhere. 我的笔丢了，我哪儿也找不到它了。
I won't be back tonight. Please tell my wife about it .我今晚不回来了，请你向我妻子说一声。
I was disappointed with the film. I had expected it to be much better. 我对这部电影很失望，我曾盼望它更好。
Tom's mother kept telling him that he should work harder, but it didn't help.汤姆的妈妈不停地告诉他要努力，但这没起作用。
2、用作指示代词（demonstrative it）
相当于this或that，it有时不特指某件东西，而代表前面已提到的或将会发生的某件事情。
-Who is knocking at the door? —谁在敲门？
-It's me. —是我。
I had a talk with the student. It was very helpful to her. 我和那个学生谈了次话，对她非常有帮助。
It happened during my stay in the United States. 事情发生在我在美国的时候。
-Whose exercise book is that? —谁的作业本？
-It's his . —是他的。
3、用作非人称代词（impersonal it）
代词it可用来指除人以外的一切生物和事物，无阴阳性之分。可指时间、距离、度量、价值、自然现象（天气、气体、阴暗等）。
-What's the time? —“几点了？”
-It's half past ten. —“十点半”（指时间）
It is late autumn now. 现在是深秋。（指时间）
It is rather cold today, isn't it?今天很冷，是不是？（指天气）
-How far is it from here to the station? —从这儿到车站有多远？
-It's about two kilometers. -大约两公里。（指距离）
It is raining hard. 雨下得很大。（指自然现象）
-What's the cost of the dictionary? 那本词典多少钱？
It is sixty-three. 六十三元（指价值）
It did not snow much last winter. 去年冬天雪下得不多。（指自然现象）
三、it作引导词
1、作形式主语（formal subject）
当主语是动词不定式短语、动词的-ing形式短语和主语从句时，为了避免头重脚轻，往往把主语放在谓语动词之后，习惯上用it作形式主语来指代后面的真实主语。这个it称为引导词（anticipatory）it，作形式主语，放在谓语动词后的主语是真正的主语。
(1) 代替不定式短语
Is it necessary to complete the design before National Day? 国庆节前完成这个设计有必要么？
In fact, it is a hard job for the police to keep order in an important football match.事实上，在重要的足球比赛时，警察维持秩序很困难。
It's not easy to finish the work in two days. 两天之内完成这个工作不容易。
It is better to build houses on rock than on sand. 把房屋建在岩石上比建在沙地上要好。
It is necessary to use a short-wave radio.使用短波收音机很有必要。
It is not a good habit to stay up late.开夜车不是个好习惯。
（2）代替动词-ing形式短语
It is no use crying over spilt milk.覆水难收。
It is dangerous walking on thin ice.在薄冰上行走是危险的。
It won't be any help my going with you.我跟你去也没什么帮助。
Is it any good trying again?再试一次有用吗？
（3）代替主语从句
It is a fact that English is being accepted as an international language.事实上，英语是公认的国际语言。
It is a pity that you didn't go to see the movie.你没去看这个电影， 真是可惜。
Has it been decided when we are to hold the sports meet?我们啥时候开运动会决定了没有？
It is known to all that Christopher Columbus discovered America in 1492.众所周知，克里斯托夫•哥伦布于1492年发现了美洲。
Does it matter if he can't finish the job on time?如果他不能按时完成那工作要紧吗？
2、作形式宾语(formal object)
当复合宾语中的宾语是动词不定式、动词-ing形式短语、宾语从句时，往往把宾语放在它的补足语的后面，而把引导词it放在全句谓语动词和宾语补足语的中间。放在宾语补足语后面的宾语叫真正宾语，放在全句谓语动词和宾语补足语中间的引导词it叫形式宾语。
（1）it代替不定式短语
I don't think it possible to master a foreign language without much memory work. 我认为不进行大量的记忆学好英语是不可能的。
He feels it his duty to help others.他感到帮助别人是他的职责。
She found it very difficult to answer the question.她发现很难回答这个问题。
People in the west make it a rule to buy Christmas presents for their relatives and friends.西方国家的人们习惯为他们的亲戚朋友买圣诞礼物。
The little boy found it very interesting to study English.那小男孩发现学英语很有趣。
(2）it代替动词-ing形式短语
I think it no use telling them.我认为告诉他们没用。
Do you consider it necessary sending more people over?你觉得再派一些人去有必要吗？
We think it a waste arguing with him.我们认为和他争吵是浪费时间。
(3)it代替从句
I hate it when people talk with their mouths full我讨厌人们说话时嘴里吃东西。
They want to make it clear to the public that they do an important and necessary job.他们想向公众表明，他们所做的工作既重要又有必要。
We think it necessary that we (should) attend the meeting.我们认为我们去参加那个会议很有必要。
They found it strange that no one would take the money.他们感到奇怪，谁也不要这笔钱。
四、it用在强调结构中
当我们要强调句子的某一部分（通常总是主语，宾语或状语）时，常用强调结构。其形式为“it is (was)+被强调的部分+who(that)+句子的其它成分”。在这种结构中，it无实际意义，它只帮助改变一个句子的结构，使某一成分受到强调。如I told her the news at the gate yesterday. 这个句子就可借助it改为下列几种形式，各强调一个不同的成分。
1、强调的成分
（1）强调主语
It was I that told her the news at the gate yesterday. 昨天在大门口告诉她这个消息的是我。
It is the ability to do the job that matters not where you come from or what you are.是你的能力很重要，而不是你从哪来或者你是谁。
（2）强调宾语
It was her that I told the news at the gate yesterday.昨天在大门口我告诉消息的是她。
It was a pen that he bought in the shop yesterday.他昨天在哪家商店里买的是一支笔。
(3)强调地点状语
It was at the gate that I told her the news yesterday. 我昨天是在大门口告诉她这个消息的。
It was in the bookstore that I met your brother the other day．（上海1990）
前几天我是在书店遇到你哥哥的。
(4)强调时间状语
It was yesterday that I told her the news at the gate. 我是昨天在大门口告诉她这个消息的。
It was not until 1920 that regular radio broadcasts began.直到1920年才开始正规的收音机广播。
It was not until she took off her dark glasses that I realized that she was a famous film star. 直到她拿下墨镜我才认出她是著名的影星。
It was about 600 years ago that the first clock with a face and an hour hand was made. 早在600年前第一座带有表面和时针的钟就造出来了。
2、使用“It is/was…that”强调句型要注意的几点
（1）被强调部分指人时，其后除了用that之外，也可用who或whom。强调的是主语时，使用who；强调的是宾语时，使用whom。
It was Mary who picked up the wallet. 是玛丽拾到了那个钱包。
It was one of my old schoolmates whom I visited yesterday. 我昨天去看的是我的一位老校友。
It was Jack that I met in the park last week.我上星期在公园遇到的是杰克。
It was her that I met in the park yesterday.昨天在公园时里我遇到的是她。
(2)强调句的时态
一般说来，原句的谓语动词如果是现在或将来各种时态，则用句型“It is .... that(who, whom) ... 。”如果原句谓语动词是过去各种时态，则用句型“It was ...that(who, whom)....”
It was only when I reread his poems recently that I began to appreciate their beauty.只是在最近我在读他的诗时才开始欣赏到它的美。
It is Smith who is leaving for Beijing tomorrow.是史密斯先生明天去北京。
(3)在强调时间、地点、原因或方式状语时，不能用when、where、why或how，只用that。
It was because her mother was ill that she didn't go with us.就是因为她母亲病了，她才没跟我们一起去。
It was in Shanghai that the Chinese Communist Party was founded on July 1, 1921.中国共产党是在上海于1921年7月1日建立的。
注意：可用“It is/was because…that…”结构强调because引导的原因状语从句，如上例所示，但不能用该结构强调由since或as引导的原因状语从句，如不能说:It is everybody is here that let's begin our discussion.也不能说: It was as it was raining hard that they had to stay at home.
（4）强调“not...until”结构
在强调“not...until”结构中由until短语（或从句）表示的时间状语时，要用固定的强调句型“It is/was not until ... that ... 。” 其中that从句中的谓语动词用肯定式。
My father did not come until 12 o’clock last night.
It was not until 12 o'clock last night that my father came home.昨晚直到十二点我父亲才回家。
It was not until his father came in that the boy began to prepare his lessons．直到她爸爸进来时，那个男孩才开始复习功课。
It was not until she came to see us that we knew her mother was ill in bed．直到她来看我们时，我才知道她妈妈卧病在床。
（5）在强调一般疑问句中的某一成分时，主句要用一般疑问句的语序。
Did this happen in Guangzhou?
Was it in Guangzhou that this happened? 此事是在广州发生的吗？
Was it in 1969 that the American astronaut succeeded in landing on the moon？是在1969年美国宇航员成功登上月球的吗？
（6）特殊疑问句中只有疑问词可以强调，其强调结构是：“疑问词+is/was it +that … ？”
When did you get to know her?
When was it that you got to know her?你是在什么时候认识她的？
（7）可以强调方式状语和以because引导的原因状语从句。
It was unwillingly that he did it for me. 他是不情愿地替我做的那件事。
It was because he was ill that we had to come back early.正是因为他病了，我们才不得不早归。
（8）可以强调宾语补足语，当连系动词不是be，表语部分是名词性词组时，也可使用这种结构强调表语，但是，当连系动词是be时，不能强调表语。
It is white that they painted the house. 他们把房子漆成的是白色。（强调宾语补足语）
It is a chief engineer that he becomes now. 他现在担任的是总工程师。（强调表语）
（9）强调句的谓语动词除了可采用现在时和过去时的单数形式外，有时根据需要还可采用复杂的形式。
It must have been Mary that you saw just now. 你刚才看到的一定是玛丽。
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