“If you saw the photograph of the man, would you be able to identify him?” In the sentence, a hypothetical situation is expressed. This is expressed with the use of "if" in the first clause. "would" and not "will" is used since the situation because it is a hypothetical situation and not something that actually happens. "would" and not "could" is used because the use of "could" would be redundant or unnecessary because the ability to identify "him" is already expressed with the use of "be able to".