It is certain/sure that....// I'm sure/ certain that....
1. It is certain that he will succeed.
2. I'm certain that he will succeed.
3. It is sure that he will succeed.
4. I'm sure that he will succeed.
I think sentence 3 is not grammatical and sounds awkward?
Re: It is certain/sure that....// I'm sure/ certain that....
I agree that it is unnatural. Although "sure" and "certain" are synonyms, "sure" is more subjective than "certain". That is why it works better when it applies to a person's belief or opinion (I am sure) then when it takes on a broader meaning (It is sure).
Originally Posted by wotcha
See usage note here: sure - definition of sure by the Free Online Dictionary, Thesaurus and Encyclopedia.