it 特指,就这个东西
one 泛指,这类东西
比如 there are some bikes in the shop and I want to buy one.
I find this red pen very nice and I will buy it at once.
至于that 和 this是一般出现在文章中的,代替上下文中的某件事或者某物.that 既可以代替前文也可以代替后文,而this只能代替前文