that指前文所提过的同类事物,一般用于比较句中,如：The weather in Beijing is different from that in Shanghai.
it指前文所提到的同一个事物,如：I hda a nice pen,but I lost it.
one指的是前文所提到的同类事物中的一个,同类而不同一,如：I had a nice pen,but I lost it yesterday.So I bought a new one this morning.