"It" 用法及其句型和固定搭配，是高中英語語法的重點、難點，又是近幾年高考的熱點，因此應給予充分的重視，現將it用法歸納如下：
一、It用作實詞
表達以下概念:指代前文提到的事物，前文中的this, that;替代前文中的內容;指代一位性別不明的小孩或未知的人;指代未指明但談話雙方都知道的那件事;指代時間、天氣、氣候、距離等自然現象……
二、It用作形式主語
替代作主語的從句、動詞不定式、動名詞，而把真正作主語的從句、動詞不定式、動名詞置於句尾。
It 作形式主語的常見句型：
1. 代作主語的動詞不定式，其句型為
(1) It be adj. (for sb.) to do sth.
此處adj. 通常為描述事件的形容詞：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.
此處adj. 通常為描述人的形容詞：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替代作主語的動名詞的常見句型
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替代作主語的從句常見句型
(1) It is + noun +從句
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作主語的句型
1. It takes sb. … to do…(=sb takes…to do…)某人用多長時間做某事
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)像某人做某事的風格
例 It was (just) like him to think of helping us.
3. It's (about/high) time that… should /v-ed…是該做某事的時候了
例 It's(about/high) time that we should take action.
4. It's the x-th time (that) … have v-ed…第幾次做某事了
例 It's the third time that he has failed the driving test.
5. It is/has been… since …continuous v-ed(延續性動詞) 某動作已有多長時間不發生了
例 It's 10 years that he lived here
6. It was(not)… before…過(不)了多長時間某動作發生了
例 It was not long before they arrived.
四、It 作形式賓語
用來替代作賓語的從句、動詞不定式、動名詞，而把真正作賓語的從句、動詞不定式、動名詞置於句尾。
It 作形式賓語的常見句型：
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.