形容词加that是什么从句

是主语从句。例如

lt is  good that we are going to play football next Monday.(下周一我们去踢足球太好了。)       这个句子的it从形式看是主语，实际上它只是代替后面that引导的从句，这个从句才是真正的主语(主语从句)。这是因为主语从句作主语太长，放在前面显得头重脚轻，才用it代替它。

It's+形容词+that+从句. - ...... 这个属于主语从句的范畴：

It+be+said/reporeted/believed+that从句 it是形式主语，指代后面的that从句

如：

It's certain that you should leave.

I am glad that you chose to write on this topic.我很高兴你选择了这个话题。

又如：

I am happy（主句） that she will be back（从句）.

类似的同位语从句：

I heard the news that our team had won.我听到了我们队赢了的消息。

主句：I heard the news

从句：that our team had won

类似的还有It句型：

It is doubtful that they will put much pressure on companies to boost returns.

值得怀疑的是，它们是否会对企业施加很大压力以提高回报率。

一般为主语从句

形容词后面加that，常用形式有：

①it is important that…重要

②it is impossible that…不可能…

It is impossible that he should go home.

他不可能会回家去。

③it is necessary that…有必要…

It is necessary that I should return it this morning. 

今天早上我就有必要还回去。

④it is advisable that…最好…

It is advisable that everyone should have a map. 

最好每一个人都有一张地图。

⑤it is essential that…务必…

It is essential that the meeting starts at eight. 

会议务必在8点钟开始。

⑥it is desirable that…最好…

It is desirable that everything should be ready by five o’clock. 

最好一切在5点前准备就绪。