一，that 用作指示代词 that 用作指示代词， this 相对， 跟 分别表示远指和近指， 它们的复数形式分别为 those 和 these.
Eg. This is a pen and that is a pencil. These are some pens and those are some pencils.
尤其要注意的是 this 要指代后面要讲的事，而 that 则指代前面已提到的事情。
Eg. What he told me is this: he wanted to go to Beijing University. He didn’t attend the meeting . That is because he was ill.
That 和 those 还常用于表示对比的句子中指代比较对象。That 指代不可数名词和单数名词， those 则指代复数名词。
Eg. The population of china is larger than that of Japan. These machines are better than those we produced last year.
二，that 引导定语从句 that 引导定语从句，既可指代人，也可指代物，在定语从句中充当主语，表语，或宾语，作 主语时不可以省略。
Eg. The man that is talking with our headmaster is my father. The book that my father gave me as a birthday gift is gone. The city is no longer the one that it used to be.
特别注意以下几种情况只能用 that 作引导词：
1． 当先行词前有形容词最高级、序数词以及 both,all,every, no,any,little,much,some,very,the only 等词修饰时。
Eg. This is the most interesting film that I’ve ever seen. This is the last lesson that we have this term. And then I watched all the glasses that wre on the table fell off onto the floor.
2. 当先行词为 all,everything,nothing,anything,little,much 等不定代词时。
Eg. Everything that should be done has been done.
3. 当先行词中既有人又有物时。 Eg. They talked about the things and persons that they could remember at school.
4. 当主句是以 who 或 which 开头的特殊疑问句时。 Eg. Who is the woman that is wearing a red coat.
5. 当先行词在定语从句中作表语时。 Eg. Tom is no longer the person that he used to be.
还要注意以下两种情况中不能用 that. 1. 非限制性定语从句中用 which,不用 that Eg. He came late again, which made his teacher angry. 2. 介词后的关系代词用 which, 不用 that Eg. This is the book for which I paid twenty dollars.
That 作为连接词可引导主语、表语、宾语以及同位语从句，其特征为：无实义，不充当句 子成分，引导主语从句时不可以省略。
Eg. That he came late made me angry. (主语从句)
I think that you’’ll have a bright future.（宾语从句）
The trouble is that we can’’t collect enough money.（表语从句）
The news that he had won the first prize made us very happy.（同位语从句）
1． That 引导定语从句是可充当主语、宾语或表语，而引导同位语从句时不充当句子成分。 2． 定语从句对前面的名词起修饰限定的作用，而同位语从句则说明该名词的具体内容，起 解释说明的作用。 Eg. The suggestion that she made is of great value.她提出的那个建议是很重要的。 The suggestion that he should not go there is of great value.他不应该去那儿的这条建议很重要。
有时候，为了强调句子中的某个成分，常用强调句式：it+be+被强调部分+that+句子的其他 部分。 Eg. It is I that am to blame.(此句中为了强调主语 I， 将其放到了被强调位置， 句子的其他部分 则放到了 that 的后面)。 这个结构可用来强调除谓语以外的其他句子成分如主语、宾语、表语、状语等。
Eg. I met him in the street yesterday. It is I that/who met him in the street yesterday.（强调主语） It is him that/whom I met in the street yesterday. （强调宾语）
It is in the street that I met him yesterday. （强调地点状语）
It is yesterday that I met him in the street. （强调时间状语）
1． 强调部分表示人的时候，可用 who 或 whom 来代替 that.
2． 强调含有 not……until 结构的句子时，要用 it+be+not until+that 结构，that 后的句 子必须为肯定句,且用陈述语序。 Eg. He didn’t return to his hometown until after liberation.
It was not until after liberation that he returned to is hometown.他是解放后才回 到家乡的。
3． 判断一个句子是否是强调句的方法：将句子结构 it be ……that 去掉，如果剩余部分仍 为一个完整地的句子，则该句为强调句；否则，该句是 it 作形式主语的主语从句句型。 Eg. It is true that he once went to Canada.(it is ……that 不可去，否则该句不成立， 说明此句为主语从句句型) It is on December 11,2001 that China became a member of WTO.(去掉 It was ……that， 原句依然成立，此句为强调句)
4． 与 that /who 引导的定语从句的区别。仔细分析 that /who 在句中的作用，如果它们可 有可无，则为强调句，否则为定语从句。 Eg. It was in the lab that was set up by Mr. Smith that they finished the experiment.(第一个 that 在句中作主语，不可省略，该句为定语从句；第二个 that 在句中 不作任何成分，该句为强调句) 是在史米斯先生建立的实验室，他们完成了实验。
五， that 用作程度副词，其含义相当于 so, 在课文中也经常出现。 Eg. I can’t run that much。= I can’t run so much.
Though we don’t earn that much money.= Though we don’t earn so much money.
I was that scared I didn't know what to do.
I would have walked out, I was that angry...
It's interesting that you like him...
Well I'm not going, and that's that.
That's it, carry on!
That's it, the fire's out now.
That's it, I've had enough!
So that's it─the fuse had gone.
You don't love me any more, is that it?