it作形式主语和形式宾语it既可作人称代词，用来表示动物或无生命的物体等，也可以作无人称代词，在句中作主语，表示“时间”、“距离”、“天气”等。
它还可以引导强调句，使语气得以加强。
此外，it在句中能作引导词，充当句子中的形式上的成分，如形式主语、形式宾语等。
㈠it作形式主语：it作形式主语时没有具体的意义，而只是帮助把真正的主语移到句子后部去，使句子显得平稳一些。
it作形式主语时，可以代替三种形式：不定式、动名词和主语从句。
1. it代替不定式短语常用于下列句型中：It + be +形容词+ (for/of sb.) to do sth.It + be + 名词+ to do sth.It takes sb. some time to do sth. 干…花了某人…时间It’s up to sb. to do sth. 干…是某人的职责或义务如：①It is everyone’s duty to obey the law.遵守法律是每个人的义务。
（it作形式主语, 代替不定式短语to obey the law）②It is difficult for a foreigner to learn Chinese.对于一个外国人来说，学习汉语是困难的。
（it作形式主语, 代替不定式短语to learn Chinese）③It is not right to use these places as rubbish dumps.把这些地方用来倒垃圾是不对的。
（it作形式主语, 代替不定式短语to use these places as rubbish dumps）④It took them a year to build the bridge.建这座桥花了他们一年的时间。
（it作形式主语, 代替不定式短语to build the bridge）⑤It is bad manners to stare at people.瞪着眼睛看人是不礼貌的。
（it作形式主语, 代替不定式短语to stare at people）⑥It is up to us to help those in need.帮助那些有困难的人是我们的责任。
（it作形式主语, 代替不定式短语to help those in need）⑦It is not up to you to tell me how to do my job.我怎样干我的工作不用你来多嘴。
（it作形式主语, 代替不定式短语to tell me how to do my job）◆ It be + 形容词+ for sb. to do sth.与It be + 形容词+of sb. to do sth.：如果句型中的形容词描述的是sb.的品质、品格，在逻辑上可以作sb.的表语，则sb.前应用介词of，否则就用for。
如：①It’s n ecessary for us to use a short-wave radio to pick up the programmes.我们用一台短波收音机收听这些节目是必要的。
②It’s important for us to learn English well.学好英语对我们来说是重要的。
③It’s kind of you to help me.感谢你对我的帮助。
（= You are kind to help me. ）④It was foolish of him to give up the job.他放弃那工作是愚蠢的（= He was foolish to give up the job.）⒉ it代替动名词短语：动名词作主语用后置形式的情况远不及不定式那么普遍，常限于no good, no use, useless, waste等词的后面。
①It is no good/ use having a car if you can’t drive.如果你不会开车，有车也没用。
( it作形式主语,代替动名词短语having a car)②It is a waste of time watching TV.看电视是浪费时间。
（it作形式主语,代替动名词短语watching TV）③It is no use asking him.问他没有用。
（it作形式主语,代替动名词短语asking him）④It is no use talking to him about it.和他谈这事没有用。
（it作形式主语,代替动名词短语talking to him about it）⒊ it代替主语从句：这个主语从句可以用连词that引导，也可以用连接代词或连接副词引导。
如：①It was clear that they had no desire for peace.很清楚他们没有和平的诚意。
（it作形式主语,代替主语从句that they had no desire for peace）②It is doubtful whether she will be able to come.她能否来还很难说。
（it作形式主语,代替主语从句whether she will be able to come）（it作形式主语,代替主语从句what ③It was clear enough what she meant.她的意思是够清楚的。
she meant）④It hasn’t been made clear when the new road is to be opened to traffic.还没明确宣布新路什么时候通车。
（it作形式主语,代替主语从句when the new road is to be opened to traffic）⑤It is a mystery to me how it all happened.这都是怎么发生的对于我来说还是一个谜。
（it作形式主语,代替主语从句how it all happened）用it作形式主语，把真正的主语从句移至后面的常见句型有：◆ It is a pity/ a shame/ a fact/ an honour/ good news/ no wonder/ a wonder… that…如：①It was a pity that the engineer couldn’t come.可惜工程师没能够来。
②It is a wonder that he continues to gamble when he always loses!令人惊讶的是他一直输还一直赌。
③（It is）no wonder (that) you were late!难怪你来晚了。
④It is a shame that the rain spoiled our picnic.真可惜，这场雨把我们的野餐给搅了。
⑤It is a fact that English is being accepted as international language.英语被认为是国际语言，这是一个事实。
◆ It is + 形容词（如：strange/ natural/ surprising/ obvious/ true/ fortunate/ wonderful/ funny/ possible/ impossible/ likely/ unlikely/ clear/ unusual/ certain/ important…）that …如：①It is certain that he will win.他一定会取胜。
②It is important that we should learn from each other and help each other.我们应当互相学习，互相帮助，这是很重要的。
③It was obvious that the child had been badly treated.很显然那个孩子受过虐待。
④It is likely that he will ring me tonight.他今晚可能给我打电话。
⑤It is strange that he should have left without telling us. 真奇怪，他也没说一声就走了。
⑥It is unlikely that their group will get ahead of us. 他们小组不会赶到我们前头去。
◆ It seems/ happened/ turned out/ suddenly struck me/ occurred to me…that …如：①It happened that I wasn’t there that day. 恰好那天我不在那里。
②It seems that his idea is more practical.看起来他的意见更实际一些。
③It suddenly occurred to me that I knew how to solve that problem.我忽然想起我知道怎样解决那个问题。
◆It + be + 过去分词（如：said/ reported/ announced/ decided/ expected/ hoped/ known/ believed…）that…如：①It is hoped that the number of deer will greatly increase very soon.(相当于：People hope that the number of deer will greatly increase very soon)人们希望鹿的数目将会很快增加。
②It is said that nothing has been done about it. 据说至今对此没采取任何措施。
③It is reported that more than twenty people were killed in that bus accident.据报道在那次的公共汽车事故中有二十多人丧生。
④It is known that he is one of the best teachers in our school.大家都知道他是我们学校最好的教师之一。
⑤It is believed that the flood there is the biggest in 100 years.据认为那儿的洪水是百年来最大的洪水。
* 注意要在It is suggested/required/desired/ordered/proposed…that…的主语从句中用虚拟语气：(should) + 动词原形如：①It is requested that he give a performance at the party.有人请求他在聚会上表演一个节目。