1、主语从句可直接位于主语的位置，如果从句较长，谓语又较短，可用it作形式主语，而将从句放在句末。常见的句型有：
It is a facta pitya questiongood news that...
It seemsappearshappenedhas turned out that...
It is clearimportantlikelypossible that...
It is said eportedestimatedhas been proved that...
It is said that comic books create a connection between people of the same generation.
It seems that the performance is very useful.
2、what引导的主语从句表示“...的东西时”，一般不用it作形式主语。
What we lack is experience.
3、what,who,when,why,whether等词含有各自的疑问意义，但它们引导的主语从句，都用陈述语序。
How the plan is to be carried out should be discussed again.
I did know why I felt like crying.
From:https://www.zgghmh.com/CET-4/20000447.asp