it的用法及练习
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。
