一.用that不用which

　　1）先行词是最高级或他前面有形容词最高级时

　　Thisisthebest--that--hasbeenusedagainstairpollutionincities.

　　Englishisthemostdifficultsubject--that--youwilllearnduringthesedays.

　　2)先行词是序数词或它前面有一个序数词时

　　Heisthelastperson--that--iwanttosee.

　　3)主句中有疑问句

　　Whichisthepen--that--youlost?

　　4)先行词既有人又有物时

　　Thebikeanditsrider--that--hadrunoveranoldmanweretakentothepolicestation.

　　5)先行词是all,much,little,something,everything,anything,nothing,theone时

　　6）先行词前有only,any,few,little,no,all,oneof,thesame,thevery等词装饰时

　　7）有2个定语从句,其中一个关系代词已用which,另一个应用that.

　　Tombuiltupafactorywhichproducedthingsthathadneverbeenseenbefore.

　　二.用which不用that：关系代词前有介词

　　Thisistheschoolin--which--youwillstudy.

　　.原句本来就有that

　　Letmeshowyouthebook--that--iborrowedfromthelibrary--which--wasnewlyopen.

　　哎,没分的,打那么辛苦白干了...T-T