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定语从句定语从句在句中做定语，修饰一个名词或代词，有时也可以修饰部分或整个句子。被修饰的名词，词组或代词即先行词。定语从句通常出现在先行词之后，由关系词（关系代词或关系副词）引出。关系代词有：who,whom,whose,that,which,as。关系副词有：when,where,why,how。关系代词和关系副词放在先行词和定语从句之间，起连接作用，同时又可做定语从句的一个成分。当关系代词做宾语时可以省略。定语从句中的谓语动词必须在人称上和数量上和先行词保持一致。定语从句分为限制性定语从句和非限制性定语从句。1、关系代词引导的定语从句1)who,whom,that这些词代替的先行词是人的名词或代词，在从句中所起作用如下：Ishethemanwho/thatwantstoseeyou?（who/that在从句中作主语）Heisthemanwhom/thatIsawyesterday.（whom/that在从句中作宾语）2)whose用来指人或物，（只用作定语，若指物，它还可以同ofwhich互换），例如：Pleasepassmethebookwhose(ofwhich)coverisgreen.3)which,that它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等，例如：Aprosperitywhich/thathadneverbeenseenbeforeappearsinthecountryside.（which/that在句中作宾语）Thepackage(which/that)youarecarryingisabouttocomeunwrapped.(which/that在句中作宾语)关系代词that和which都可以指物，that和Who都可以指人，其用法区别：不用that的情况：a)在引导非限定性定语从句时(错)Thetree,thatisfourhundredyearsold,isveryfamoushere.b)介词后不能用Wedependonthelandfromwhichwegetourfood.c)多用who的情况①关系代词在从句中做主语Afriendwhohelpsyouintimeofneedisarealfriend.②先行词为those,people时Thosewhowereeitherfoolsorunfitfortheirofficescouldnotseethecloth.③先行词为all,anyone,ones,one指人时Onewhodoesn'tworkhardwillneversucceedinhiswork.④在Therebe句型中Thereisastrangerwhowantstoseeyou.⑤在被分隔的定语从句中AnewteacherwillcometomorrowwhowillteachyouGerman.⑥在有两个定语从句的句子中，其一用who，其二用that,但若先行词后接两个以上的并列定语从句时，后一个必须重复前一个关系代词。Thestudentwhowaspraisedatthemeetingisthemonitorthatisverymodestandstudiesveryhard.Thereisateacherwhoisalwaysreadytohelpothersandwhoenjoyswhathedoes.2)只能用that作为定语从句的关系代词的情况a)在不定代词，如：anything,nothing,everything,all,much,few,any,little等作先行词时，只用that，不用which。Allthatisneededisasupplyofoil.Finally,thethiefhandedeverythingthathehadstolentothepolice.b)先行词有theonly,thevery,thejust修饰时，只用that。Heistheverymanthathelpedthegirloutofthewater.c)先行词为序数词(thelast)、数词、形容词最高级时，只用that。ThefirstEnglishbookthatIreadwasThePrinceandthePauperbyMarkTwin.d)先行词既有人，又有物时。Hetalkedabouttheteachersandschoolsthathevisited.e)当主句是以who或which开始的特殊疑问句时，用that以避免重复。Whoisthepersonthatisstandingatthegate.f)关系代词在从句中做表语Heisnotthemanthatheusedtobe.2、关系副词引导的定语从句关系副词可代替的先行词是时间、地点、方式或理由的名词，在从句中作状语。关系副词when,where,why,how的含义相当于介词+which结构，因此常常和介词+which结构交替使用，例如：Thereareoccasionswhen(onwhich)onemustyield.Beijingistheplacewhere(inwhich)Iwasborn.Isthisthereasonwhy(forwhich)herefusedouroffer?I'msurprisedthewayhow(bywhich)heworksouttheproblem.注意：①在非限制性定语从句中，介词+which结构不能代替关系副词。如：Theysetupastatefortheirown,wheretheywouldbefreetokeepNegroesasslaves.②含有介词短语的动词一般不能拆开，介词仍放在动词后面。Isthisthebookwhich(that)shewaslookingfor?3、名词/数词/代词/形容词最高级+介词+关系代词引导定语从句Shehaswrittenabook,thenameofwhichIhaveforgotten.Therearefifty-fivestudentsinourclass,allofwhomareworkinghard.Therearefivecontinentsintheworld,thelargestofwhichisAsia.4、as,which引导非限定性定语从句的差别由as,which引导的非限定性定语从句，as和which可代整个主句，相当于andthis或andthat。As一般放在句首，which在句中。Asweknow,smokingisharmfultoone'shealth.Thesunheatstheearth,whichisveryimportanttous.as可引导非限制性从句，常带有“正如”的意思。Asisknow,smokingisharmfultoone'shealth.用法区别：(1)as引导的定语从句可置于句首，而which不可。Asweallknow,heneversmokes.(2)as代表前面的整个主句并在从句中作主语时，从句中的谓语必须是系动词；若为行为动词，则从句中的关系代词只能用which。(3)非限定性定语从句中出现expect,think,suppose等表示猜测、想象、预料等时。Shesucceededinherdoingtheresearchwork,asweexpected.(4)As的用法thesame…as;such…as中的as是一种固定结构,和……一样……。(5)Ishouldliketousethesametoolasisusedhere.Weshouldhavesuchadictionaryasheisusing.