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.
*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.
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
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.
It will be not long before he finishes his job.
It will be half a year before I come back.
It was long before they met again.
It won't be long before you regret for what you’ve done.
14. It happens (seems, appears) that句式
It (so) happened that he met an old friend of his in the street.
It seems that it will rain this afternoon.
15. It doesn’t matter whether (if, wh-word) 句式
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.
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
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.
He hates it when people use his bike.
I hate it if you say such things in public.
I’d prefer it if I didn’t have to do so much work.
I would appreciate it very much if you would help me with it.
I don't love it when she tells me how to do things.
24. S+V+ it + that句式
I take it you have been out.
You can put it that it was arranged before.
I like it that you came.
Rumor has it that the defence minister will soon resign.
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.
Look to it that this doesn’t happen again.
You may rely on it that he’ll come to meet you.
You may depend on it that he will turn up in time.
I can’t answer for it that the boy is honest. (=I can’t answer for his honesty).
I can swear to it that this man stole our money.