Would

When you are talking about the present or the future and you imagine something that is untrue or unlikely, use the past tense (‘lived’) in the if clause (NOT would): ‘If I knew her address, I’d send her a gift.’ ‘If I won a lot of money, I’d buy a new car.’
When you are talking about the past and you imagine something that is impossible, use the past perfect tense (‘had caught’) in the if clause (NOT would have): ‘If he hadn’t got on the plane, he would still be alive.’
When you are talking about the present or the future and you imagine something that is possible but unlikely, use if … should (NOT if … would): ‘If anyone should need me, I’ll be back in half an hour.’