1. it, that常常用来代替或避免重复某个名词。例如：

Where did you find your watch? - I find it in our classroom.

The population of China is larger than that of any other country.

2.that只指物，不指人，可替代可数名词或不可数名词，具有“特指”性质。指的是同类事物中的另一样东西，即同类异物。如：The library of our school is bigger than that of yours.(that代替的是另一个图书馆。)

3.That的复数形式those代替可数名词的复数。可指人或物，一般后面跟修饰语。如：

①Today's cellphones are smaller than those(=the ones)used in the past.

②The students who do best in exam are not always those(=the ones) with the best brains.

4.the one(ones)或 that ,those 指代某一名词做定语从句的先行词时，其关系代词which在that之后，which不能省略，而在the one、ones 之后,which可省略。如：

①Their problem today is somewhat similar to that which they faced many years ago.

5.代替不可数名词时，应该用that。如：The equipment is different from that.

6..it代替上文所提到的名词时，是指同类事物中的同一样东西，即同类事物。如：

I have lost my pen. I'm looking for it.

7.it和that可代替前面整个句子的意思，而one却不能；that可引导一个限定性定语从句，代替某个先行词（单复数名词均可），it和one则不能；it可代替不定式、动名词或从句，用作形式主语或形式宾语，也可用于强调句型中，而one和that则不能。如：

①-I will make bed for you. –It is very kind of you.

②Jim said he had read my novel.But that was not possible.

③A clock is an instrument that tells time.

④The students that surf the Internet will be punished by the teacher.

⑤It is nice to see you again.

⑥It is no use calling her up.

⑦I found it impossible for me to pass the test.

⑧It was I who broke your window.