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1、Unit21语法一、虚拟语气条件状语从句种类及用法条件句类型条件句主句零条件句表示普遍真理和客观事实一般现在时一般现在时第一条件句依据将来事件A,B事件很可能在将来发生一般现在时一般将来时第二条件句与现在事实相反If+主语+动词的过去式（be动词用were）主语+should/would/could/might+动词原形与将来事实相反1.If+主语+动词过去式主语+should/would/could/might+动词原形第三条件句与过去事实相反If+主语+had+过去分词主语+should/would/could/might+havedone1．零条件句。表示普遍真理和客
2、观事实。其主句、从句都用一般现在时IfyouwatchtoomuchTV,youeyeshurt.Ifweeattoomuchsweet,yourteethbecomeweak.Ifmyheartstopsbeating,Idie.2．第一条件句。依据将来事件A,B事件很可能在将来发生。主句用一般将来时，从句都用一般现在时。即：主将从现。IfIenterTsinghuaUniversity,mymumwillbuyaBMWforme.IfmyEnglishteachercriticizesmetomorrow,Iwillhaveatalkwiththeheadmaster.
3、3．与现在事实相反的虚拟语气eg.IfIwereagirl,Ishouldbeabeauty.IwouldbuytwocarsifIhadonemillionyuan.4．与将来事实相反的虚拟语气eg：IfIshouldtakeShenzhouTentothemoon,IwoulddancewithChang’E.IfIweretobecomePresidentoftheUnitedStates,Iwouldnotstartawar.5．与过去事实相反的虚拟语气eg：Ifhehadseenyouyesterday,hewouldhaveaskedyouaboutit.Ify
4、ouhadcomeearlier,youwould(might)havemethim.Ishould(would)havecalledyouifIhadknownyourtelephonenumber.6.混合虚拟条件句以上虚拟语气的句子都有固定形式，不难掌握。难就难在准确判定与事实相反的时间类别。混合虚拟条件句，即从句与主句的动作不是同时发生。这时就要分别根据各自发生的时间，选用相对的虚拟形式。eg：Ifhehadfollowedtheteacher’sadvice,hewouldnotbesoheart-brokenrightnow.与过去事实相反与现在事实相反IfCh
5、inahadnotbeenliberated,theworkingpeoplewouldstillleadamiserablelifenow.与过去事实相反与现在事实相反7.用介词短语等代替if虚拟条件句。有时虚拟条件不用if从句表示，而是用介词短语with,without,butfor或副词otherwise,连词表示。eg24：Withyourassistance(=Ifwehadyourassistance),wemightfinishtheplanearlier.如果有你的威胁，我们会早一点完成计划。IcouldnothavelivedthroughChristma
6、swithoutgivingyouthatpresent.如果没给你买那件礼物，我圣诞节都会过不好。Iwasbusylastweek,otherwiseIwouldhavecometoseeyou.上周我很忙，否则我会去看你的。8.省略if后的倒装如果if条件从句中谓语部分含有were或者had,should词。把were或者had,should放到主语前面，可以省去if。所表达的意思和带有if的句子意思相同。eg：IfIwereagirl,Iwouldwearshorthair.如果我是女生，我就留短发。=WereIagirl,Iwouldwearshorthair.If
7、IshouldtakeShenzhou10tothemoon,IwouldinviteChang--Otohaveadancewithme.=ShouldItakeShenzhou10tothemoon,IwouldinviteChang--Otohaveadancewithme.IfPeterhadn’tfalleninlovewithCuihua,hemighthavebeenadmittedtoakeyuniversity.=HadPeternotfalleninlovewithCuihua,hemighth
