(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.
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（覆水难收）.
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
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 porced.(=The couple are said to have gotten porced.)
(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.
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) 某动作持续了多长时间
例: It's 10 years since 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.
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) attend the conference.
4. verb + it+ as+ noun/adj.+ clause (verb=accept, regard, take, see, view)
例: The lecturer takes it as encouraging when so many students attend his lecture.
5. v. +it + prep. + that…
owe it to sb. that…把…归功于…
leave it to sb that…把…留给某人去做
take it for granted that …想当然
keep it in mind that…
例: Don't bother to arrange anything. Just leave it to me to sort out.
6. It用在不能直接跟宾语从句的动词后面，尤其是表示好恶的动词后，enjoy, like, love, dislike, resent, hate, don't mind, be fond of, feel like, see to 宾语从句紧跟it之后
例: I hate it you can swim so well and I can't.
7. It用在不能直接跟宾语从句的介词后面，宾语从句紧跟it之后（except that例外）
例: I'm for it that you will follow their advice.