IL EST IMPORTANT DE /IL EST FACILE DE or A

Question

My teacher says there are rules for using the subject sentences.  I can't find them anywhere.

Accepted Answer

Not quite sure what you are looking for, but have a look at an answer to a somewhat related question recently : 
https://progress.lawlessfrench.com/questions/view/why-is-a-used-in-one-place-but-de-in-the-other