[Grammar] It is certain/sure that....// I'm sure/ certain that....

[wotcha]

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?

 

[MikeNewYork]

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?

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).
See usage note here: sure - definition of sure by the Free Online Dictionary, Thesaurus and Encyclopedia.