It作形式主语常见句型动词不定式，动名词或主语从句作主语时，通常把它们放在谓语动词前，而用it作形式主语，放到句首，常见句型有：1.It+be+形容词+主语从句，如：ItisuncertainwhetherhecancometoJenny’sbirthdaypartyornot.2.It+be+名词词组+主语从句，这类名词有：afact,apity,ashame,anhonour，aquestion等，如：It’sapitythatyoumissedtheexcitingfootballmatch.Itremainsaquestionwhetherhewillcomeornot.3.It+be+过去分词+主语从句，这类过去分词有：announced,believed,expected,hoped,decided,reported,said,shown等，如：Itisreportedthat16peoplewerekilledintheearthquake.Itisnotdecidedwhowillgivetheoperationtothepatient.4.It+不及物动词(seem,appear,happen等)+主语从句,这类不及物动词有：appear,happen,seem等，如：Itseemedthathedidn’ttellthetruth.IthappenedthatIwasoutwhenhecalled.5.It+be+形容词+forsb.）+动词不定式这类句型常用形容词easy,difficult,hard,important,possible，wise等作表语，有时候为了强调不定式动作的执行者，常在不定式前加forsb，如：It’snecessaryfortheyoungtomastertwoforeignlanguage.Itisunwisetogivethechildrenwhatevertheywant.这类形容词有：easy,difficult,hard,important,possible，wise6.It+be+形容词+ofsb.+动词不定式，这类形容词常是表示心理品质,性格特征的形容词，如：kind,nicestupid,clever,foolish,polite,impolite,silly,selfish,considerate等。某人和这些形容词可以构成主系表结构。如：It’sverykindofyoutohelpmewiththework.=Youarekindtohelpme.Itseemedselfishofhimnottosharehisdictionarywithothers.=Heseemedselfishnottosharehisdictionarywithothers.7.It+be+名词词组+动词不定式，如：Itisnotagoodhabittostayuptoolate.8.It+be+名词或形容词+动名词，这类名词和形容词常常是：good,nogood,nouse,awasteof,useless,senseless等，如：It’sawasteoftimetalkingtoheranymore.Itisnousearguingaboutthematterwithhim.9.It+take(sb.)+时间（金钱）+动词不定式，如：Ittooktheworkersalmostthreeyearstofinishbuildingthedam.Itwilltakeawholedaytogettothetopofthemountainonfoot.实战演练：1.Itisgenerallyconsideredunwisetogiveachildheorshewants.(NMET1997)A.howeverB.whateverC.whicheverD.whenever2.Infactisahardjobforthepolicetokeeporderinanimportantfootballmatch.(2001上海)A.thisB.thatC.thereD.it3.isafactthatEnglishisbeingacceptedasaninternationallanguage.(NMET1995)A.ThereB.ThisC.ThatD.It4.Isnecessarytotakeoffourshoeswhenweenterthelab?(MET88)A.everyoneB.thisC.herD.it5.________willtakeyoualotoftimetofindhiminthecrowd.A.ItB.IC.WeD.They6.Itisnousehis_________there,thesituationishopelessnow.A.togoB.tobegoingC.goingD.havinggone7.Itisnevertoolatetolearn,________?A.isitB.isn’titC.doesitD.doesn’tit8.___________isknowntoallthatShenzhouVILaunchedSuccessfully.A.AsB.WhatC.ItD.That9._______isreportedinthenewspaper,theEarthquakeineastChina'sJiangxiProvincekilled13peopleA.ItB.AsC.ThatD.What10.___________certainthathisinventionwillmakepeople’slifeconvenient.A.That’sB.ThisisC.It’sD.What’s11._________thatthereisanotherfootballmatchontheairthisevening.A.ItsaysB.ItwassaidC.ItissaidD.Whatwassaid12.__________thattheirdaughtergotanopportunitytostudyabroad.A.It’saexcitingnewsB.ThisisanexcitingnewsC.ThisisexitingnewsD.It’sexcitingnews参考答案：1——5BDDDA6——10CACAC11——12CD