that引导的名词从句的用法

答：that 放在一个陈述句的前面，引导一个名词性从句如：that the earth goes around the sun 地球绕太阳转（这件事）。光是这个一个从句，你是看不出它究竟是主语从句、宾语从句、表语从句还是同位语从句的。只前将它放入主句中，才可能判断它是什么名词性的从句：
1. 主语从句：That the earth goes around the sun is known to us all.
2. 宾语从句: The teacher said that the earth goes around the sun.
3. 表语从句: One of the greatest discoveries was that the earth goes around the sun.
4. 同位语从句：The fact that the earth goes around the sun is known to us all.
判断的方法就是看这个从句中主句中作什么成分，如果是主语，则that是引导主语从句的，如果作宾语，则是引导宾语从句的。
从形式上看，出现在句首的That，一般就是引导主语从句的连词(例1），出现在句子的中间，及物动词后边的，是引导宾语从句的连词（例2）。出现在中间，系动词后边的，则是引导表语从句的连词（例3），出现在fact, news, 等后面的则为引导同位语从句的连词（例4）。
要注意的是，有时为了避免主语太长，放在前面给人感觉头重脚轻，可以用形式主语it来放在句首做个样子，而将真正的主语（从句）后置如：
It is known to us all that the earth goes around the sun.

主语从句相当于一个主语 应该在句子的开头 或者放在it be +形容词 + that 从句， it be +名词短语+that 从句。it +be + 过去分词 +that 从句。主语从句that 一般不省略。
that 引导的宾语从句 作及物动词的宾语可以省略 但做介词的宾语不能省略。

（1）连词that引导主语从句时，不在从句中做任何成分，that没有词义，不能省略。但若在非正式文体种，又用于it作形式主语的结构中，只要句子不太复杂，that可以省略。例如：
That he was chosen made us very happy.他被选中使得我们非常高兴。
That he will get two gold medals is certain.它将获得两块金牌是肯定无疑的。
It’s a pity (that) you missed the sports meeting last week.很遗憾你没有参加上周的运动会。
（2）在宾语从句中比较麻烦：
①由that引导的宾语从句,如果从句较短，其连词that可以省略。如：
Everyone could see (that) he was frightened.每个人可以看出，他感到害怕。
I think (that) he’ll be all right in a few days.我想，他过几天就会没事的。
He said (that) all the students help each other.他说所有的学生都应该互相帮助。
但在较长的句子里，如果that从句与主句谓语动词被分隔，或者有两个以上的从句时，that不可省略。
Everyone could see who should be punished and that things will improve.
每个人都明白谁应该受罚，可惜乔治受委屈了。
He announced, believe it or not, that he would never forgive her.他宣布，信不信由你，他绝不会原谅她。
如果在mention, declare, report, state等动词后面时，that通常也不省略。例如：
The mayor declared that he would attend the coming celebration.市长宣布他将参加即将举行的庆祝会。
They reported that they had seen a star in the East.他们报告说他们在东方看到一星座。
②在某些句型中，特别是带复合宾语的句子中，that引起的从句常移到后部去，前面用一个it作形式上的宾语。如：
I took it for granted that they were not coming.我想他们准是不会来了。
We all thought it a pity that their team lost the game.
他们队输掉了这场比赛，我们都感到遗憾。
③用that引导的从句作介词的宾语时是很少的，只有在except, in, but等介词后面偶尔可能用到。
I know little about him except that he lives here.除了知道她住在这儿外，我对他一点都不了解。
He differed from other people in that he always looked farther ahead in his work.
（3）由that引起的表语从句，that只其引导作用，不在从句中作成分。例如：
The trouble is that she has lost his address.问题是她把他的地址弄丢了。
The difference between water and ice is that while water is liquid, ice is sold.
水和冰之间的区别在于，水是液体而冰是固体。
My idea is that we should get more comrades to do the work.我的看法是，我们应该多弄些人手做这工作。