必须用that 1.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the very,the only,the just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the same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
定语从句用that不用who