1.it 指上面出现的东西 I found something on the desk.It was a pen.(it指something)
2.that特指单数名词,但并非上文出现的那个
The weather in Nanjing is hotter than that in Beijing.(that= the weather但并非前面的weather)
3.one = a + 前面出现的名词
There are so many interesting books in the store.I want to buy one (one = a book)