The speaker is using the second conditional form, which means he uses the simple past tense ("were") in the "if" clause, and "would" (or other modal verb) + present simple verb in the other clause. In this case, "You'd (You would) only use..." is the other clause, and "if you were sure..." is the "if" clause.
This also falls into the category of the subjunctive form, but that's too difficult to explain here!