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)