用名词说不清的事情，一定要用名词性从句！因为可以说清楚呀。
名词性从句是名词的加长版，通常分为两部分：
连接词（that）+句子（he is afraid of needles)。
PS：括号内为例子。
例如：
The secret is unbelievable. 这秘密真让人难以置信。
What is the secret ? 啥秘密？
It is that he is afraid of needles. 就是他怕打针呀！
总和：
That he is afraid of needles is unbelievable. 他怕打针，真让人难以置信。
名词性从句一般有４种用法：
作主语，作宾语，作表语，作同位语。
作主语：
That he is afraid of needles is unbelievable. 他怕打针，真让人难以置信。
That he is afraid of needles = the secret.
作宾语：
We don't know that he is afraid of needles. 我们都不知道他怕打针。
That he is afraid of needles = the secret.
作表语：
The fact is that he is afraid of needles. 实际上他怕打针。
That he is afraid of needles = the secret.
is为be动词，be动词后面接表语。
作同位语：
The fact that he is afraid of needles is unbelievable. 他怕打针这个事实真是让人难以置信。
That he is afraid of needles = the secret.
从句放在名词后面，作为名词的同位语。
over。
