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Grammar•it在句中的作用有两个，一是作代词，二是作引导词．作代词时，通常指代前面提到过的人或事物，或表示季节，气候，距离等．作引导词时，一般用来充当形式主语，形式宾语或用于强调结构，现分述如下：e.g.Itisimpossible(forsofewworkers)todosomuchworkinasingleday.Itwasfoolishofhertowastemoneyonsuchclothes.2)It+be+adj./n.+clause.e.g.ItiscertainthatTomwilldowellintheexam.e.g.Itisimpossible(forsofewworkers)todosomuchworkinasingleday.Itwasfoolishofhertowastemoneyonsuchclothes.2)It+be+adj./n.+clause.e.g.ItiscertainthatTomwilldowellintheexam.I.作代词1.用作人称代词，代替前文所提到的事物Mypenismissing.Ican’tfinditanywhere.Jackisill.Haveyouheardaboutit?Thefrogisnotawarm-bloodedanimal.It’sacold-bloodedone.2.用以代替指示代词this,that--What’sthis?-----It’saflag.Whoseexercise-bookisthat?----It’sherse.g.ItwilltakethemmorethantenyearstocompletetheThreeGorgesDam.5)It+be+said/reported/told等+that-clause.e.g.ItissaidthathewillleaveforBeijingtomorrow.e.g.ItwilltakethemmorethantenyearstocompletetheThreeGorgesDam.5)It+be+said/reported/told等+that-clause.e.g.ItissaidthathewillleaveforBeijingtomorrow.3.It作人称代词，指代婴儿或身份不明的人。E.g---Whoisknockingatthedoor?---It’sme.---Whoismakingsuchanoise?---Itmustbethechildren.•4.表示天气,时间,距离,金钱，日期，价值，度量，温度，环境情况等•It’stimeforlunch.•Whenspringcomes,itgetswarmerandwarmer.•It’sabout2,000kilometersfromheretothezoo.•ItisMarch29thtoday.•Itwasgettingdark.•ItrainsalotinNewZealand.•Itissome3,500kilometresfromPolynesiatoNewZealand..Ⅱ.作引导词１．作形式主语或宾语it作形式主语常用于：Itbe+adj./n./p.p+不定式/动名词/主语从句①.代替不定式Toseeyouagainisapleasure.=Itisapleasuretoseeyouagain.Todrivetoofastisdangerous.=Itisdangeroustodrivetoofast.TomasterEnglishisnecessaryforus.=ItisnecessaryforustomasterEnglish•注:主要结构有1)Itis/was+adj/n.(for/ofsb)todosth．在此结构中,当形容词是表示不定式逻辑主语的特征或属性(如:wise,bright,kind,nice等)时,介词用of;形容词用来说明不定式短语(如:important,possible,necessary,strange等)时,介词要用for。e.g.Itisimpossibleforsofewworkerstodosomuchworkinasingleday.Itwasfoolishofhertowastemoneyonsuchclothes2)It+take+(sb.)sometime/somemoney+todosth.e.g.ItwilltakethemmorethantenyearstocompletetheThreeGorgesDam•②.代替动名词作主语•主要结构有It’snouse/good/uselessdoingsth.Eg.It’snousecryingwhenyouhavedifficulties.It’snousetryingtorepairtheship.Thereisnoneedtowaitforher.•③.代替整个句子作主语•主要结构有:•1)Itbeclear/certain/possible/true…+that…•Itispossiblethatoneofthemkeptakindofbankwheretheworkerscouldkeeptheirmoneysafe.•ItisprobablethattheChineseteamwillwinthatfootballgame.•2)Itbe+important/necessary/right/strange/natural/curious+that…（虚拟语气，should可省)•Itwasobviousthatthemanwasnotonthespotatthetimethemurdertookplace.•Itisnecessarythatadoctorbesentforatonce.•3)Itbeapity/shame/fact/myduty…that…eg.ItisafactthatEnglishisbeingacceptedasaninternationallanguage.ItisagoodthingthatNewZealandhelpstheMaoritokeeptheirownlanguageandculture.4)It+be+pp/v.+ing+that从句Itbesaid/reported/broadcast/announced/known/learned+that…e.g.Itisnotsurprisingthathefailedintheexam.ItissaidthathewillleaveforBeijingtomorrow.5)Itbesuggested/recommended/required/demanded…+that…+(should)+do•It’snotyetknownwhereshehasgone.•Itisknownthattheearthtravelsaroundthesun.•It’sreportedthatthetroopshavecrossedtheborder....