泛指可数名词单数用one,复数用ones
特指可数名词单数用the one(=that),复数用the ones(=those)
it指代前文提到的可数名词单数或者不可数名词,也常用来指代心目中想知道的人或者物(例如1.--what's that ?---It's a book。2. Who is knocking at the door?---it's me)
当指代前文提到的不可数名词时,不能用one或者the one而要用that(例如:The weather here is hotter than that in the north)