one it that的区别

one = a +上文出现的名词
it 用来指代上文出现的单数名词（（必需是单数名词,包括可数名词单数和不可数名词））
that= the +上文出现的名词（必需是单数名词,包括可数名词单数和不可数名词）
that不是用来指代上文的名词,而是另外一个.
比如：There are many bags.I want to buy one.= I want to buy a bag.
I saw a dog on my way to school.It was big.it指上文的那条狗.
The weather in Nanjing is hotter than that in Beijing.that=the weather,但并非指上文的天气,而是另外一个天气.