1）当先行词是或被序数词,最高级,不定代词修饰时,关系词用that不用which.
　　2）当先行词既有人又有物时,用that.
　　3）当先行词带有the only,the very,the same,the last,the one等词时,用that不用which.
　　4）当主句中有who或which时,为避免重复用that.