形容詞加that是什麼從句
是主語從句。例如
lt is  good that we are going to play football next Monday.(下週一我們去踢足球太好了。)       這個句子的it從形式看是主語,實際上它只是代替後面that引導的從句,這個從句才是真正的主語(主語從句)。這是因為主語從句作主語太長,放在前面顯得頭重腳輕,才用it代替它。
It's+形容詞+that+從句. - ...... 這個屬於主語從句的範疇:
It+be+said/reporeted/believed+that從句 it是形式主語,指代後面的that從句
如:
It's certain that you should leave.
I am glad that you chose to write on this topic.我很高興你選擇了這個話題。
又如:
I am happy(主句) that she will be back(從句).
類似的同位語從句:
I heard the news that our team had won.我聽到了我們隊贏了的訊息。
主句:I heard the news
從句:that our team had won
類似的還有It句型:
It is doubtful that they will put much pressure on companies to boost returns.
值得懷疑的是,它們是否會對企業施加很大壓力以提高回報率。
一般為主語從句
形容詞後面加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.
他不可能會回家去。
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還回去。
④it is advisable that…最好…
It is advisable that everyone should have a map. 
最好每一個人都有一張地圖。
⑤it is essential that…務必…
It is essential that the meeting starts at eight. 
會議務必在8點鐘開始。
⑥it is desirable that…最好…
It is desirable that everything should be ready by five o’clock. 
最好一切在5點前準備就緒。