1.当先行词为all,little,much,few,everything,none
等不定代词时关系代词用that而不用which.Iwilltellhimall
thatyoutoldmeattheball.
Therearefewbooksthatyoucanreadinthisbookstore.
2.当先行词既有人又有物时,关系代词用that,不用which.
Heaskedaboutthefactoriesandworkersthathehadvisited.
3.当先行词有形容词最高级修饰时,关系代词用that,而不用which.
ThisisthebestnovelthatIhaveeverread.
GuilinisoneofthemostbeautifulcitiesthatIhaveevervisited.
4.当先行词有序数词修饰时,关系代词用that,不用which.
ThefirstsightthatwascaughtattheGreatWallhasmadealasting
impressiononhim.5.当先行词被theonly,the
very,thesame,theright等修饰时,关系代词用that,不用which.
Thatistheverythingthatwecando.
Itistheonlybookthatheboughthimself.
6.当先行词被all,every,any,much,little,few,no等修饰时,关系代词用that,不用which.
Youcantakeanyroomthatyoulike.
Thereisnoclothesthatfityouhere.
7.当主句是以which开头的特殊问句时,定语从句关系代词一般用that而不用which.
Whichofthebooksthathadpictureswasworthreading?
8.在强调句型"Itis...that..."中,只用that,不用which.
Itisinthisroomthathewasborntwentyyearsago.
9.在"such(thesame)...as..."句型中,关系代词要用as,而不用which.
Weneedsuchmaterialsas(notwhich)canbearhightemperature.
10.表示"正如...那样","正象..."之意时,用as,不用which.
Marywaslateagain,ashadbeenexpected.
11.As引导的定语从句可以放在主句前面,也可以放在主句后面或主句中间,而由which引导的定语从句只能放在主句后面。
Asweallknow,oceanscovermorethan70%oftheearth.
usedtodo："过去常常"表示过去习惯性的动作或状态，但如今已不存在。
Motherusednottobesoforgetful.
Scarfusedtotakeawalk.(过去常常散步)
beusedtodoing：对……已感到习惯，或"习惯于"，to是介词，后需加名词或动名词。
Heisusedtoavegetariandiet.
Scarfisusedtotakingawalk.(现在习惯于散步)
usedto的用法
Weusedtogothereeveryyear.(我每年都去那儿。)
Thisusedtobeashabbyhouse.(此房年久失修)
usedto,would这两个词语都可以表示过去常做某事，有时可以换用。
usedtodo强调整过去习惯性的行为或状态，但是现在没有这种行为或状态了。因此，这个短语的内涵是今昔对比。
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