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。
形容词加that是什么从句
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。
形容词加that是什么从句
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。