"that"的指代用法如下：
特指同类事物
"that"用于特指前面提到的同类事物，但不是同一个。例如：
The weather in Beijing is colder than that in Shanghai.
Here is the book you asked for. That one on the table is mine.
指代不可数名词
"that"可以指代前面提到的不可数名词。例如：
The weather in Beijing is colder than that in Shanghai.
指代可数名词单数
"that"也可以指代前面提到的可数名词单数。例如：
Here is the book you asked for. That one on the table is mine.
指代较远的事物或人
"that"通常用于指代离说话者较远的事物或人。例如：
Look at that building over there!
That day, I met an old friend.
引导从句
"that"还可用作关系代词，引导从句，并在从句中充当句子成分。例如：
The book that I borrowed is very interesting.
The problem is that we don't have enough time.
强调句
"that"还可用在强调句中，强调句子中的特定成分。例如：
It was at that moment that I realized I was in trouble.
总结：
"that"主要用于特指同类事物，可指不可数名词或可数名词单数。
"that"用于指代离说话者较远的事物或人。
"that"可用作关系代词，引导从句，并在从句中充当成分。
"that"还可用在强调句中，强调句子中的特定成分。