It 既是代词，也是引导词（语法亦叫功能词），是最简单也是用法最复杂的一个词。作为代词，it主要用于指代前文提及的某种事物、某种情况，甚至还可以指性别不明确的人；也可以指时间、日期、地点、距离、天气、温度、环境等。作为引导词，it可以用作形式主语、形式宾语以及引导强调句型等。关于it作为代词的用法，大家都已经十分熟悉。本文不再赘述，而主要对it作为功能词的用法，做一归纳和总结。
1. It is necessary (for sb.) to do sth. 句式
在该句式中，it是形式主语，动词不定式是真正主语，不定式的逻辑主语用for sb.。句式中的形容词通常表示不定式的重要性、必要性、难易程度等基本情况。如 important, necessary, natural, easy, safe, common, normal, hard, difficult, dangerous, unusual, rare, impossible, pleasant等 。例如：
It is very important for us to take part in physical training out of class.
It is too difficult for a three-year-old child to answer such a question.
不可以说：
*We are very important to take part in physical training out of class.
*A three-year-old child is too difficult to answer such a question.
2. It is kind (of sb.) to do sth. 句式
该句式与上述句式十分相似，形容词往往表示不定式逻辑主语的品质或特征属性，所以用of sb., 不用for sb. 常见的这类形容词有：good, bad, brave, careless, clever, cruel, foolish, honest, horrible, kind, lazy, modest, naughty, nice, polite, rude, silly, stupid, wise, wrong,等。例如：
It is very kind of you to help me out of trouble.
It was brave of him to jump into the river to rescue the drowning boy.
上述两句可以转换为：
You are very kind to help me out of trouble.
He was brave to jump into the river to rescue the drowning boy.
3. It is no good (use, pleasure, fun) doing sth. 句式
该句式中，it是形式主语，表语部分是否定意义，形容词往往表示“益处，用处，乐趣”等含义，真正主语一般用动名词形式。例如：
It is no good learning English without speaking English.
It is no fun climbing mountains on raining days.
4. It is clear (apparent, obvious, true, probable, likely, possible, impossible, certain…) that句式
该句式中，it 是形式主语，真正的主语是that 引导的主语从句，这是it引导主语从句最常见的一种结构。
It is very clear that he’s round and tall like a tree. = That he’s round and tall like a tree is very clear.
It is very likely that he will not consent.= That he will not consent is likely.
5. It is important (necessary, right, strange, natural...) that句式
该句式跟上一个同属一种类型。由于主句中的形容词性质不一样，它们表示事物的重要性、必要性、正确与否以及带有“出乎意料的语气”，所以，that 从句中要用虚拟语气（should + 动词原形）。建议让学生记住用于该句型的形容词。例如：
It is important that we should learn English well.
It is necessary that he should remember these language rules.
6. It is said (reported, believed, agreed, announced, hoped, expected, thought...) that句式
该句型中的it 仍是形式主语，真正主语是that 引导的主语从句。该结构常译为“据说（据报道，人们相信……）”。
It is said that he has gone to Los Angles.
It is reported that another earth satellite has been sent into orbit.
7. It is suggested (ordered, recommended, required...) that句式
该句式跟上一个同属一种类型。主句中的过去分词表示请求，建议，命令等含义，that后的从句要用虚拟语气（should + 动词原形），should 可以省略。常译为“据建议（有命令说……）
It is suggested that the meeting ( should ) be put off till next Friday.
It was ordered that the soldiers (should) arrive there in two hours.
8. It is a pity (a shame, a fact, a wonder, a pleasure...) that句式
该句式中，如果表示出乎意料，常译为“竟然”，that从句也用虚拟语气（should + 动词原形）。若没有这种意义时，则不用虚拟语气。
It is a pity that such a thing should happen in your class. 这种事竟然发生在你们班上，真是遗憾！
It is a pity that he is ill. 他生病了，真遗憾！
9. It is time (about time, high time) that 句式
该句式中，that 后的从句用虚拟语气：可以使用一般过去时态, 有时也用should + 动词原形。常译为“该是…的时候了”，意味着时间有些晚了。
It is time that children went to bed.
It is high time we began to work on the painting.
It is about time that we should have our lunch.
10. It is the first (second...) time that句式
该句式要跟上一个句型区别开来。该句型中的 that 从句不用虚拟语气，而是用完成时态。主句的谓语动词如果是一般现在时，后面从句就用现在完成时；如果是一般过去时，后面从句则用过去完成时。该结构中 that 可以省去；it有时用 this 替换。常译为“这是第一（二）次……”。
It is the first time I have been to the Great Wall.
It was the third time he had broken the rules.
11. It is...since 句式
该句式主要用于表达非延续动词和一段时间连用的问题。It is 之后是一段时间，时态是一般现在时（有时可以是现在完成时），since 引导的从句通常是一般过去时态，而且是瞬间动词。
It is five years since my older brother joined the army.
It has been a long time since we departed last time.
12. It is ...when句式
该句式中的 when 引导一个时间状语从句，主句中的 it 是代词指时间，表语由具体的时间充当。常译为“当……的时候，时间是……”。
It was 5 o’clock when he came here.
It was midnight when he returned home.
13. It be +一段时间+ before句式
该句型中，it 是代词指时间, 主句中的时态通常是将来时或过去时两种时态。主句中的表语多是long, not long, 3 days, 2 weeks 等表示一段时间。常译为“……之后就……”。
It was 3 days before he came back home.
三天后他回了家。（直译：在他回家之前时间是三天。）
It will be not long before he finishes his job.
不久后他就会完成工作。（直译：在他完成工作之前，时间是不会长的。）
【句子分析】
It will be half a year before I come back.
在我回来之前，时间将有半年。意为“我半年之后才会回来。”
It was long before they met again.
在他们再次见面前（的日子里），时间是很长的。意为“过了好久他们才又见面。”
It won't be long before you regret for what you’ve done.
在你后悔你的所作所为之前，时间不会长了。意为“不久你就会为你的所作所为后悔的。”
以上三句，It表示“时间”，before表示“在……之前”的本义，结合时态和肯定否定，就很容易理解句子的含义了。
14. It happens (seems, appears) that句式
该句式中，it是形式主语，that 引导的是主语从句，也可以理解成是一固定句型。
It (so) happened that he met an old friend of his in the street.
碰巧他在大街上见到了他的一位老朋友。
It seems that it will rain this afternoon.
今下午看起来要下雨。
15. It doesn’t matter whether (if, wh-word) 句式
该句式中whether 引导的从句是真正主语，该句型常译为“不论（是否，何时，何地什么等）……没关系……”。
It doesn’t matter whether they are old or new.
It doesn’t matter what he says.
It matters little if I miss my bus.
16. It looks (seems, appears) as if句式
该句式中it作为引导词无意义, as if 引导一个方式状语从句（一说表语从句，因为放在系动词之后）。常译为“看起来好像……”，如果与事实不相符合，则用虚拟语气。
It looks as if that old man is ill.
It seemed as if it were going to rain.
It appears as if they have lost interest.
17. It is a waste of time (money) doing sth. 句式
It’s a waste of time your talking to him.
It is a waste of money buying such an old car.
18. (It is) no wonder (that) 句式
It’s no wonder that he is not hungry; he has been eating sweets all day.
No wonder he was so pleased.
19. It makes a (no, mush) difference whether句式
It’s doesn’t make much difference whether I am there or not.
It will make a difference whether you know first aid or not.
20. It takes sb. +time+ to do sth. 句式
It took thousands of people ten years to build the palace.
It takes years to master a new language.
【说明】此句型可以有如下几种转换：
It took me an hour to write the letter.
=The letter took me an hour to write.
=I took an hour to write the letter.
21. It is up to sb to do sth. 句式
It’s up to you to make the choice.
It is up to her to do the job.
22. I think it important to do sth. 句式
该句式中的it 作形式宾语。为了记忆方便，我们称该句型为“6123结构”。
6指主句中6个常用动词：think, believe, make, find, consider, feel；1指形式宾语it ；2指宾补的两种形式：形容词或名词；3指真正宾语的三种形式：不定式短语，动名词短语或that引导的宾语从句。
This special strain of rice makes it possible to produce one-third more of the crop in the same fields.
She made it a rule to get up at six and take a walk in the morning.
Do you find it funny to see someone sliding on a banana skin,
The teacher feels it his duty to help the student with his English.
We thought it necessary that the two sides should stop arguing.
I consider it rude to stare at other people.
We believe it useless reading without standing.
23. I like it if (when) 句式
在一些表示“喜好、厌恶”的动词之后，如enjoy, like, dislike, love, hate, prefer, appreciate等，后接 if 从句或 when 从句，通常先在动词后接 it 作形式宾语。
She won’t like it if you arrive late.
她不喜欢你迟到。
He hates it when people use his bike.
他讨厌别人用他的自行车。
I hate it if you say such things in public.
我讨厌你在大庭广众之下说那样的事。
I’d prefer it if I didn’t have to do so much work.
要是我不必做那么多工作，那就太好了。
I would appreciate it very much if you would help me with it.
如果你能帮助我做这事，我会十分感激。
I don't love it when she tells me how to do things.
我不喜欢她对我做事指手划脚。
24. S+V+ it + that句式
I take it you have been out.
我猜你刚出去回来。
You can put it that it was arranged before.
你可以说这是以前安排的。
I like it that you came.
你来了，我很高兴。
Rumor has it that the defence minister will soon resign.
据传闻，国防部长不久就要辞职。
25. S+V+ prep + it + that句式
说明：能用于此结构的动词不多，常见的有see to, look to, insist on, stick to, depend on, answer for 等。
I’ll see to it that all these letters will be sent to the post before twelve.
我将注意让所有的信件在12点以前送到邮局。
Look to it that this doesn’t happen again.
注意不要再发生这种事。
You may rely on it that he’ll come to meet you.
你放心，他会来接你的。
You may depend on it that he will turn up in time.
你可以相信他会及时赶到的。
I can’t answer for it that the boy is honest. (=I can’t answer for his honesty).
我不能保证这个 男孩是诚实的。
I can swear to it that this man stole our money.
我可以发誓，这个人偷了我们的钱。