必须用that1.当everything,anything,nothing,all,none充当先行词时.

　　2.当先行词由every,any,all,some,no,little,few,much等修饰时,that常被省略3.当先行词前有序数词或最高级修饰时4.当先行词前有thevery,theonly,thejust修饰时5.当先行词本身是who,which时6.当先行词是人物混有时7.当先行词前有thesame修饰,两物为同一物时8.当先行词在从句中代替人做主补时