It做形式主语做形式主语 Its necessary for the young to master two foreign language. Its a waste of time talking to her any more. Its a pity that you missed the exciting football match. 动词不定式动词不定式动名词动名词从从 句句形式主语It 作为作为形式主语形式主语的用法的用法当当 不定式、动名词、从句不定式、动名词、从句 等用作等用作句子主语时，为保持句子平衡，通句子主语时，为保持句子平衡，通常把真正的主语放在句末，而在句常把真正的主语放在句末，而在句首使用形式主语首使用形式主语it.拒绝是令人遗憾的拒绝是令人遗憾的.记住这一点是很重要的记住这一点是很重要的.Its a pity to refuse.Its very improtant to remember this一、代替动词不定式做形式主语It +be +adj.+（for sb.）to do sth. 此处adj. 通常为描述事件事件的形容词：easy，difficult，hard，necessary，impossible， important，natural, likely，obvious，useful，useless，dangerous做某事是Its necessary for us to master a foreign language.It +be+ adj.+ of sb. to do sth 此处adj. 通常为描述人人的形容词：kind，nice，rude，cruel，careless，foolish，stupid，wise，crazy某人做某事，某人是eg. Its foolish of you to drive a car after drinking. Its very kind of you to help me with my English.It + be +名词词组+动词不定式eg. It is not a good habit to stay up too late. 常见句型：It takes/took sb. some time/money to do sth.It is no good crying over spilt milk.It is no use casting pearls before swine .覆水难收覆水难收不要明珠暗投，对牛弹琴不要明珠暗投，对牛弹琴构成如下结构：构成如下结构：It is +no use/no good/not any good/not any use+ doing sth.二、代替动名词作形式主语It + be + 名词词组 + 主语从句这类名词有：a fact, a pity, a shame, an honour，a question，a surprise，common knowledgeeg.Its common knowledge that people cannt live without water . 三、代替从句做形式主语It + 不及物动词 + that（as if）-从句这类不及物动词有：look，seem，appear，happen，occureg.It seems that he does not tell the truth.It is time +that sb. +v-ed/ should doEg. Its time that you went to bed Its time that you should go to bed.Its necessary/ impossible/ important/ natural that sb. (should) do Eg. Its important that we (should) keep the balance of nature.It + be +adj.+that-从句It + be +-ed（过去分词）+ that-从句It is said that 据说It is reported that 据报道It is supposed that 据推测It is hoped that 希望It is well known that 众所周知It is generally considered that普遍认为It is suggested that 有人建议练习：把下列格局转换为it做形式主语的句子1.He was rude to say so.2.Trying again is of use.3.When and where to hold the party hasnt been decided.It was rude of him to say so.It is of use trying again. It hasnt been decided when and where to hold the party.Thank you .