一、that 作限定词或代词

that可以作限定词或代词连接名词或名词短语，也可单独使用，用以指代人或者物。此外，that不仅可以指代单独的人或物，还可以指代一句话。

that有时可用作替代词，用来代表前已提过的事物，以避免重复，如：

1. A grandparent’s job is easier than that of a parent. 祖父母的任务比父母要轻松一些。
2. A dog’s intelligence is much greater than that of a cat. 狗的智慧比猫高很多。
3. The dialect in one town may he quite different from that of the next town. 一个城镇的方言可能与另一个城镇很不相同。
4. The weather of Beijing is just as cold as that of Lanzhou, but colder than that of Shanghai. 北京的气候和兰州一样冷，但比上海冷。

二、that作副词表示程度，相当于so或very much

1. It isn't all that cold. 天气还没有那么冷。
2. It’s about this high. 大概这麽高。
3. Is it this hot every day? 每天都这么热吗?
4. I didn’t think it would be this easy. 我没想到会这样容易。
5. I didn’t realize she was that ill. 我没想到她病得那么重。
6. The book is about this thick. 那本书大约有这么厚。
7. When you pay that much for a meal you expect the best. 一顿饭花那么多钱是想吃到最好的东西。

三、that作连词引导各类从句

that引导名词性从句

that后面连接一个完整的陈述句，可以位于主语、宾语、表语或同位语的位置，分别引导主语从句、宾语从句、表语从句和同位语从句。此时，that没有实意，而且不在从句中做任何成分。that引导主语从句和同位语从句时不可以省略。

That the plates are moving is now beyond dispute. (主语从句)
地球板块是在漂移的，这一事实毋庸置疑。

注意that which的用法

that which中的which为关系代词，用以引导定语从句，其意为“……的事或物”，这是一种比较正式的用法，它通常可以用what代之。如：

1. That which (=What) many people say is not always true. 很多人说的话并不表示一定是真的。
2. That which (=What) I say and do is my own affair. 我说什么做什么是我自己的事。
3. They say the only real knowledge is that which can be measured. 据说唯一真实的知识是可以检验的知识。