I.it／one ／that三者均可用作代詞,指代前面提到的名詞.一般說來,it指代同名同物; one與that則指代同名異物.
I have lost my umbrella; I'm looking for it.（該句中it就是指前面的my umbrella）
I have lost my umbrella; I think I must buy one.（one在該句中表泛指,因為my umbrella已經丟了）
The umbrella you bought is cheaper than that I bought.（替代詞that在該句中特指“theumbrella I bought”,以區(qū)別“the umbrella you bought”）
II.one與that雖可用來指代同名異物,但one為泛指,相當于a／an＋名詞; that為特指,相當于the ＋名詞.所以one所指代的名詞的修飾語一般為 a／an ／some ／any; that所指代的名詞的修飾語往往是the ／this ／that.
A chair made of steel is stronger than one made of wood.（該句中one可以換成a chair）
The water in the cup is hotter than that in the pot.（該句中that可以換成 the water）
III.one只能代替可數名詞單數,代替可數名詞復數時用ones; that既可以代替不可數名詞也可以代替可數名詞單數,代替可數名詞復數時用 those.
I like this pen more than that one.（one代替可數名詞單數pen）
There were a few young people and some older ones in the house.（ones代替可數名詞復數people）
Mary's handwriting is far better than that of Peter.（that代替不可數名詞 handwriting）
These pictures are more beautiful than those.（those代替可數名詞復數 pictures）
IV.one既可代替事物,也可代替人,that只能代替事物而不能代替人.有時可以用the one或the ones代替that或those.
The one ／That on the table is mine.（該句中The one代替事物,并且也可以用That）
He is the teacher,the one who is loved by the students.（該句中the one代替人,不能用that）
He advised the farmers to choose the best seed-heads,the ones ／those that had the best color.（該句中the ones代替事物,并且也可以用those）
V.one一般有前置修飾語,有時也可有后置修飾語或不用修飾語.而 that不能有前置修飾語,但可有后置修飾語.
Cook was a strict but good captain,one who took good care of his sailors.
The water in the well is cleaner than that in the river.
VI.it可以替代句中的不定式或從句等,充當形式主語或形式賓語.one與that均無此用法.
It is known to everybody that the moon travels around the earth once every month.
I found it hard to get on with her.
VII.it與that均可以替代上文全句的內容或部分內容,而one ／ones則不可以.
He has saved my life; I'll never forget it.（it代替第一分句）
Tom is painting his house.I am told he does it every four years.（it代替前面分句的部分內容painting his house）
Let's say we meet here at three o'clock.That ought to give you time to buy everything.（That代替前面表述的內容）