“this”、“that”既可以作形容词,又可以作指示代词；而“it”只能作代词.one可用数词又可作代词.
指“物” 
1.“this”指近物,“that”指远物,“it”没有远近之分.如：
This is a computer.这是一台电脑.（在近处） 
That is a bike.那是一辆自行车.（在远处） 
2.在回答“this”和“that”作主语的疑问句时,常用“it”代替“this”和“that”.如：
－Is this/that a car?这/那是一辆小汽车吗?
－Yes,it is./No,it isn′t.是的,它是./不,它不是.
－What′s this/that?这/那是什么?
－It′s a ruler.这/那是一把尺子.
3.陈述在一起的两样东西时,应先说“this”,后说“that”.如：
This is a book.That is a pen.这是一本书,那是一枝钢笔.
4.one与it的区别 
one指代上文提到过的同类事物中的一个,但不是上文提到的那一个事物.如果指同类事物中的一些,要用ones.如：
This apple is small.Please give me a big one.
这个苹果小,请给我一个大的.
These books are mine.Those ones are Lily's.
这些书是我的,那些书是莉莉的.
it指代前文中提到过的那一个事物.如：
My bike is very old,but I like it very much.
我的自行车很旧,但我很喜欢它.
Where is my new pen?I can't find it.
我的新钢笔在哪儿?我找不到它了.

it代替上文出现过的同一个东西,I have a bike,it is new. 
one代替上文出现过的同类单数,She has an old bike,I have a new one 
this指靠近你的那个.that指远离你的.This is my bike,that is hers. 
the是在第二次提到时用.That is a bike,the bike is mine

that,those代指在前文中出现的对等的部分（不是同一个事物,只是对等）,that指代可数名词单数及不可数名词,those指代可数名词复数
如：The weather in Beijing is colder than that in Kunming.
The apples in red basket are not as sweet as those in black basket.
而it指前文提到的事物,是同一物
如：My pen was lost ,did you see it?

that,those代指在前文中出现的对等的部分（不是同一个事物,只是对等）,that指代可数名词单数及不可数名词,those指代可数名词复数
如：The weather in Beijing is colder than that in Kunming.
The apples in red basket are not as sweet as those in black basket.
而it指前文提到的事物,是同一物
如：My pen was lost ,did you see it?