用法说明一：

it和that都替代“the+单数名词(可数或不可数)”，都是特指，都可替代可数和不可数名词，但it指前面提到的“同一”事物，而that是指前面提到的“同类”事物。如：

My father bought me a pen and I like it very much.我父亲给我买了一支钢笔，我非常喜欢它。(it替代the pen，指前面提到的父亲给我买的那支钢笔)

This dictionary is more useful than that (=the one)I bought yesterday.这本词典比我昨天买的更有用。(that替代the dictionary，与前面的this dictionary是同类)

用法说明二：

one替代“a＋单数可数名词”，表示泛指；其复数形式ones替代泛指的复数名词。特指的the one相当于that，替代“the+单数名词”；the one的复数形式the ones，替代“the+复数名词”，在口语中也常用those代替。当后面有of短语时，多用that或those；当有前置修饰语时，只能用one(s)。另外，one(s), the one(s), those都只能替代可数名词。如：

Radios are useful for me to learn English. I’d like to buy one.收音机对我学英语很有用，我想买一台。(one替代a radio，是泛指收音机这类东西中的一台)

We still have shortcomings，and they are very big ones too. 我们还有缺点，而且是很大的缺点。(ones替代shortcomings)

We kept seats for those who might arrive late.我们给可能来晚的人留了座位。(those=the ones替代the persons)

Waves of red light are about twice as long as those of blue light.红色光线的波长约为蓝色波长的两倍。(those替代the waves)

The population of Scotland was eight times as large as that of Cornwall. 苏格兰的人口是康沃尔人口的八倍。(that替代不可数名词the population，不能用the one)