"It" 用法及其句型和固定搭配，是高中英语语法的重点、难点，又是近几年高考的热点，因此应给予充分的重视，现将it用法归纳如下：
一、It用作实词
表达以下概念:指代前文提到的事物，前文中的this, that;替代前文中的内容;指代一位性别不明的小孩或未知的人;指代未指明但谈话双方都知道的那件事;指代时间、天气、气候、距离等自然现象……
二、It用作形式主语
替代作主语的从句、动词不定式、动名词，而把真正作主语的从句、动词不定式、动名词置于句尾。
It 作形式主语的常见句型：
1. 代作主语的动词不定式，其句型为
(1) It be adj. (for sb.) to do sth.
此处adj. 通常为描述事件的形容词：easy, difficult, hard, necessary, unnecessary, possible, impossible, likely, unlikely, right, wrong, important, unimportant, legal, illegal, well-mannered, ill-mannered, polite, impolite, clear, obvious, certain, suitable, proper, fit, useful, useless, dangerous…
例 It is illegal (for a teenager) to drive a car without a license.
(2) It be adj. of sb. to do sth.
此处adj. 通常为描述人的形容词：kind, unkind, nice, rude, cruel, considerate, thoughtful, thoughtless, careful, careless, silly, foolish, stupid, clever, wise, crazy.
例 It’s kind of you to help me with the problem.
(3) It替代作主语的动名词的常见句型
It’s no good/use doing…
It’s (well)worth doing…
It’s (well)worth one’s while doing/to do…
It’s (well)worth while doing/ to do
例 It’s no use crying over spilt milk.
2. It替代作主语的从句常见句型
(1) It is + noun +从句
例 It is no secret that the president wants to have a second term at office.
(2) It is adj. +clause
It’s surprising that… (should)………竟然……
It’s a pity/shame that…(should)………竟然……
例 It’s important that you should apologize to her for your rudeness.(=It’s of much importance that you should apologize to her for your rudeness.)
(3) It verb sb. + clause= It is v-ing + clause
It+surprise/delight/interest/disappoint/worry/disturb/annoy/amaze
/bother/concern/frighten/please/anger sb. that…
例 It worried me that she drove so fast.(= It was worrying that she drove so fast.)
(4) It verb (to sb.) that…= sb/sth verb to do
(verb = appear, seem, come about, emerge, follow, chance, happen, occur, transpire, turn out , work out)
例 It (so) happened/chanced that they were out.(= They happened/chanced to be out.)
(5) It is v-ed that…=sb/sth is to do
(verb=say, report, think, believe, hope, expect, agree, accept, decide, determine, intend, plan, understand, know)
例 It is said that the couple have gotten divorced.(=The couple are said to have gotten divorced.)
(6) It is v-ed that … (should)…
(verb=demand, request, require, order, suggest, advise, recommend
例 It is suggested that they should begin with the third question.
三、It作主语的句型
1. It takes sb. … to do…(=sb takes…to do…)某人用多长时间做某事
例 It took the men a week to mend our roof.(= The men took a week to mend our roof.)
2. It’s (just)(un)like sb. to do…(不)像某人做某事的风格
例 It was (just) like him to think of helping us.
3. It’s (about/high) time that… should /v-ed…是该做某事的时候了
例 It’s(about/high) time that we should take action.
4. It’s the x-th time (that) … have v-ed…第几次做某事了
例 It’s the third time that he has failed the driving test.
5. It is/has been… since …continuous v-ed(延续性动词) 某动作已有多长时间不发生了
例 It’s 10 years that he lived here
6. It was(not)… before…过(不)了多长时间某动作发生了
例 It was not long before they arrived.
四、It 作形式宾语
用来替代作宾语的从句、动词不定式、动名词，而把真正作宾语的从句、动词不定式、动名词置于句尾。
It 作形式宾语的常见句型：
1. verb+ it+ adj./noun (for/of) to do/clause (verb=think, believe, suppose, consider, feel, make, keep…)
例 I think it hard for you to do the task on your own./I think it hard that you’ll do the task on your own.
2. verb+it+adj./noun (one’s) doing (adj.=useless/worth/worthwhile)(noun=no use/no good/worth one’s while/a waste of time/money/energy/words) (verb=think, believe, suppose, consider, feel, make, keep…)
例 I’ll make it worth your while telling me about his secret.
3. verb+it+ important/unimportant/necessary/unnecessary/natural/essential that … (should)…
verb+it+of much/great/no/little importance that…(should)…
(verb=think, believe, suppose, consider, feel, make, keep…)
例 I think it important that you (should) attendthe conference.
