at what1 (18)GRAMMAR GRAMMAR When you are using if to talk about something that might happen in the future, use the present simple tense, not will or shall• if I failthe test (NOT if I will fail the test).To refer to the present or the future after ifwhen you are talking about something unlikely or untrue, use the past tense, not 'would' or 'should'• If someone gave me the money, I'd buy a car tomorrow (NOT If someone would give me the money...)!! In formal English or in writing, use were not was when the subject of the clause is I, he, she, it, there or a singular noun• If I were in that position, I'd get legal advice. • Imagine how you would feel if your child were killed.In normal conversation, you can also use was• If I was ten years younger, I'd go out with him.!! The expression if I were youis fixed. Do not say 'if I was you', even in normal conversation• If I were you, I'd have a talk with your parents.To refer to the past when you are talking about something that did not happen, use the past perfect tense• If he had married Laura, he would have been unhappy (NOT If he would have married Laura...).
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.