1.that:比较句中替代词,不可数名词that替 2.it:替代若系本身物,it 代单数 they 代复数 指代动物、物品,可以直接用于句中. it is hers/they are theirs 指代前面的一句话等,放在前面一句话之后紧接着的句子中,做主语用 3.有的时候通用:that/it is great! 4.one 和ones 指代前面已经说过的物品,放在前面一句话之后紧接着的句子中,做宾语用.one 指代单数物品,ones 指代复数物品 one泛指或者指代前句的each已经说明是单数形式 The apples here are very small,this is the biggest one. ones (those后+名词复数) girls in this class are beautiful,those ones are the hottest! 5.the one(特指,后面为定语从句)