that是代词的时候的用法

that 用作代词
1.that 用作指示代词（复数形式是those）,其指代意义同形容词用法,同时它还可以用作定语从句中的先行词；还可为了避免重复,代替前述名词.
That is what he told me.
What is that (which) you have got in your hand?
The price of rice is higher than that of flour.
2.that 用作关系代词,引导限制性定语从句.先行词可人可物,用法相当于who或which.（但是在下列情况下多用that：先行词既有人又有物时；先行词有形容词*、序数词、不定代词、very,only等修饰时；先行词是不定代词时）
He talked about the teachers and schools that he had visited.
I think it one of the most wonderful films that the film company has ever produced.
She has little information that is useful for our research.
Is there anything that I can do for you?
请注意,that 在定语从句中作宾语时通常可省略.
The books (that) I sent you will help you in your studies.