that引导名词性从句的用法

首先纠正一下：that既可以在定语从句中做定语从句的引导词，这时候可以充当主语也可以充当宾语；也可以在名词性从句中做连接词，这时候that连接的各个名词性从句（除宾语从句外）具备一个共同特点：陈述句，也就是没有任何连接词，tant这时符合“三不”：不意思（没有意思），不成分（在句子中不作任何成分），不省略（不能够省略，因为省略后句子就混乱了），关键是分清楚你所遇到的句子是定语从句还是名词性从句。下面的讲解可以参考一下（定语从句的情况省略了）：
（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。
（4）用作同位语的从句，叫同位语从句，他一般跟在某名词（如fact, news, idea, thought, question, reply, report, remark等）的后面，引导同位语从句常用连词that（没有词义，不作成分 ），连接副词how, when, where等。I had no idea that you were here.我不知道你也在这儿。
I have no idea when he will be back.我不知道他什么时候回来。
We heard the news that our team had been beaten.我们听到了我们队失败的消息。
I’ve come from Mr. Smith with a message that he won’t be able to come tonight.
我从史密斯先生哪儿来，他让我告诉你他今晚不能来了。

说的不错
定语从句 I won't forget the days ( that ) we spent together. 充当spent 到底宾语，可省 He is the right man that is always willing to help others. 指代the man 作主语
名词性从句
主语从句 That (China is a developing country is well known.) 不充当成分,不可省
宾语从句 I know ( that ) he is a good student . 不充当成分,可省
简单举例，有问题追问吧

在定语从句中，表示方式，时间或者地点的先行词后面，用that 引导定语从句。
1 方式
Can you work out a way that(in which) we can solve the problem
that 可以代表in which
2 时间
I still remember the days that we meet
3 地点
I have never been the places that the Americans lives
（以上三种定语从句中的that仅仅是关系副词，不做句子成分）
4 关系副词that 用在定语从句中还可以用来代替why，表示原因时作宾语。
I don't hear any reason that you might give me
(that 作give的宾语)当然that也可以在定语从句中作主语．
在名词性从句中，that不是关系副词，而是从属连词，名词性从句中仅仅用that 来连接有从属关系的主句和从句，不做句子成分。
名词性包括宾语从句，共有主语从句，宾语从句，表语从句，同位语从句。
这只是我的一点个人看法，希望能对你有帮助阿。