四、It用作形式主语

      It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. 替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.