that从句用法如下：
一、引导名词性从句
即引导主语从句、宾语从句、表语从句和同位语从句，that没有任何意义，也不作任何句子成分，只起引导作用。
1. 引导主语从句
由于中英语言文化的差异，汉语没有主语从句。如，“英语很重要不是人人都认同的。”汉语语法认为，“英语很重要”是主位词组做句子的主语。
而英语句子“That English is of vital importance is not acknowledged by all.”中，That English is of vital importance是主语从句。虽然that没有任何意义，也不作任何句子成分，只起引导作用，但是却不能省略。如：
That English teacher can speak fluent English is a fundamental requirement.
2. 引导宾语从句
引导宾语从句的that一般可以省略，但在以下六种情况下，一般不能省略。
（1）两个以上的宾语从句并列时，从第二个从句开始，that 不能省略。如：
Jack was informed（that）he had been enrolled in Sun Yat-sen University，and that he should register by August.
（2）介词后引导宾语从句的that 不能省略。如：
His parents expected him nothing but/except that he can make a living by himself after graduation.
（3）主句的谓语动词与that引导的宾语从句之间有插入语时，that不可省略。如：
The head teacher told Jack， after a short hesitation， that he could have a day off on condition that he could make up for the missing lessons.
（4）形容词后引导宾语从句的that一般不省略。如：
Marys parents are very content that their daughter has made such great progress in the final examination.
（5）在 it（形式宾语）+ adj.（补足语）+ that（宾语从句）结构中， that不能省略。如：
Everyone believes it strange that he could pass the driving test.
（6）在直接引语中，that引导的宾语从句与主句隔开时，that不能省略。如：
“Im sorry to say，” he said， “that you are not the suitable person for the work.”
3. 引导表语从句
表语从句位于连系动词后，引导表语从句的that一般不能省略。如：
My advice is that everyone be punctual for the appointment.
特别注意：下列句式中的that一般不能用because代替。
The reason why he was late for school is that he missed the early bus.
4. 引导同位从句
同位语从句就是在news， information， fact， word， idea等抽象名词之后，说明其具体内容的从句。引导同位语从句的that一般不能省略。如：
Based on the fact that you are enthusiastic about Chinese culture， Im writing to invite you to get involved in it.
二、引导定语从句
that常被用来代替关系代词which或who，引导定语从句。以下几种情况，多用that而不用which或who。
1. 当先行词是表示事物的all， any， anything， everything， nothing或被all， any， every等修饰时。如：
Do you have anything that you are puzzled with？
All that can be done must be done.
2. 当先行词被形容词的最高级或序数词修饰时。如：
It is one of the most touching movies that have been shown this month.
The first thing that we should do is to get some food.
3. 当先行词有the only， the very， the same， the last 等修饰时。如：
The only thing that is impressive is his unforgettable experience in Beijing.
4. 当主句以who或which开头的疑问句时。如：
Who is the girl that is delivering a speech over there？
5. 当先行词既有人又有物时。如：
The writer and his novel that you have just talked about are really well known.
三、引导状语从句
1. that 可引导目的状语从句和结果状语从句，that一般不省略。如：
The story is so touching that we will bear in our mind for a long period.
An English Speech Contest will be organized so that/ in order that students can improve their oral English.