一 it/one/that均可作代词，代指前面提到的名词
代替泛指的单数名词通常用one.如
Two heads are better than one.
I lost my old camera;this a new one
注：若one前没有形容词修饰则不能用不定冠词,但可以用another
I'm looking for a flat.I'd like a small one with a garden.
I'm looking for a flat.I'd like one with a garden.
This pen doesn't work.I must buy another one.
注意one与it的区别：
Can you lend me a pen? Sorry,I haven't got one.
Can I borrow your pen? Sorry,I'm using it？
二 it/that/the one代替表特指的单数名词
1.代替单数可数名词三者均可，代替不可数名词是不能用the one
而用it/that。表同一事物用it,表同类事物用that/the one如：
The weather here is too cold.I don't like it.
The weather of Beijing is colder than that of Guangzhou.
My uncle bought me a dictionary and I like it.
Your dictionary is more useful than the one my uncle gave me.
2.代替事物三者均可，代替人时只用the one
Who is her husband? The one by the window.
注：替代性别不明的婴儿时用it
3.当有前置修饰语时只能用the one
Which do you wang？The red one.
She would rather have the small one than the large one.
4.当有后置修饰语时，通常用the one 如：
My room is better than the one next door.
Is that the one that was published recently?
但是后置定语为of引起的介词短语，通常用that.
The populatoon of China is much larger than that of Japan.
A grandparent's job is easier than that of a parent.
one/that虽可用来指代同名异物，但one为泛指，相当于a/an+名词；
that为特指，相当于the+名词，所以one指代的名词修饰语一般为a/an/some/any.
that指代的名词修饰语往往是the/this/that.
A chair made of steel is stronger than one made of wood.
(该句中one可换成a chair)
The water in the cup is hotter than that in the pot.
(该句that可换成the water)
三 代替泛指的复数名词
代替表泛指的复数名词，通常用ones.如
Hard beds are healthier than soft ones.
Green apples often taste better than red one.
These chairs have the great advantage of being much cheaper than conventional ones.
四 代替表特指的复数名词通常用the ones
I'd like to try on those shoes.The ones at the front of the window.
Are they the ones who moved here recently?
Don't buy the expensive apples;get the cheaper ones.
注:口语中也可用those来代替特指的复数名词，尤其是当其后有of引导的介词短语或who引导的定语从句修饰时。如：
Waves of red light are about twice as long as those of blue light.
Those of you who wish to go on the trip may sign up here.
His ideas are different from those of his friends.
Students who do well in examinations are those(the ones)who ask questoins in class.
五 几点重要的补充说明
1 当替代词one/ones紧跟在形容词最高级，序数词以及th
is/that/these/those/which/either/neither/another等限定词之后时通常可以省略。如：
I think my dog's the faster(one)
Either (one) will suit me.
Let's have another (one)
She looked at each (one) carefully before she close.
Which (one) would you like? That (one) looks the nicest.
2 复数形式的ones之前一般不能直接用名词所有格，物主代词,数词以及some/any/both/several/dozen/own等词修饰。如：
Have you got any drawing-pins? Can I borrow some please?(不能说some ones)
Do you have any new diaries? We don't have any at the moment.(不能说any ones)
Do questions1 or question2,but not both .(不能说both ones)
He has three dictionaries and I have only two.
注：如果ones前有描绘性形容词修饰，则可使用上述词语。如：
误：her ones/some ones/five ones/your own ones
正：her red ones/some new ones/five bad ones/your own nice ones
在美国英语中ones不能紧跟在these/those之后，但在英国英语中可以这样用（但不常见）
3当一个名词受另一个名词修饰时，通常不宜用one(s)来替代。如：
Do you need coffee cups or tea cups?(不能说tea ones)
但若一个名词受表材料的名词修饰时，可用one(s)替代
We can lend you plastic chairs or metal ones.
one只能替代可数名词单数，替代可数名词用复数用ones。that既可以替代可数名词单数又可以替代不可数名词，代替可数名词复数用those.