that 引导什么？

1.用作关系代词,引导关系从句
2.引导名词从句
He expressed the hope that we would keep in touch with his firm.
他表示希望我们与他的公司保持联系。
My idea is that we should act on his suggestion.
我的意见是我们应当按照他的建议去办。
That she is still alive is a relief.
她还活着,这是令人感到宽慰的。
3.引导状语从句,表示原因或理由
We are glad that we have accomplished another arduous task.
我们都感到高兴因为又完成了一项艰巨的任务。
4.引导状语从句,表示目的或结果
He is working hard that he can catch up with the class.
他努力用功,以便赶上班上同学。
5.引导表示愿望,感叹的从句,主句常省略
That I could stay at home today!
今天要是我能呆在家里多好呀!

名词性定语从句。

名词性从句，用来具体说明There is no doubt 的内容的。
或者你记住固定句式：There is no doubt that....翻译成“毫无疑问....”

哎,一个问题难倒这么多英雄?
此处that引导的是同位语从句,解释doubt(名词)的具体内容