That of
使用代词的时候，首先我们考虑需不需要且能不能使用代词“those/these”， 即前面有语法正确（单复数一致）语义恰当且不模糊。作为指示代词those/these不可以“裸奔”，对Modifer进行指代，紧邻修饰。
一、that用作代词，指上文提到的人或事物(单数名词)，且需一个后置定语来修饰，此时that不可省。
Their cotton output of 1997 was double than that of 1987.
Mary's handwriting is far better than that of Peter.
二、that用作连词，在so...that, such...that, in order that 等引导的状语从句中，that不可省。
They started so early that they might arrive in time.
It is such a heavy box that nobody can move it.
He worked hard in order that he might catch up with the others.
三、that用作连词，引导主语从句时，当它放在主句谓语动词之前时，that不可省。
That he came late made his teacher angry.
That she was chosen made us very happy.
如用it作形式主语，把从句置于谓语动词之后时，that可以省略。例如：
It is a pity(that) he has made such a mistake.
It is obvious(that) he is right.
四、that用作连词，引导表语从句和同位语从句时，that不可省。
The news that our team has won the match is true.
My idea is that we should get more comrades to do t he work.
五、that用作连词，引导宾语从句时，如主句的谓语动词是及物动词make,find,see等时，且后有宾语补足语时，that引导的从句可置于宾语补足语之后，用it作形式宾语，此时that不可省。
We find it necessary that he will succeed.
He made it clear that he was not interested in this subject.
六、that用作关系代词，引导定语从句时，当它作为从句的主语时，that不可省。
All that has to do is to practice every day.
That's the same man that asked me for money yesterday.
但如that作从句的宾语时，that可省略。
The letter(that) I received was from my father.
七、在强调句型 It iswas...that结构中，that不可省。
It is English that Professor Li teaches us.
It was in Shanghai that I saw the film.