modal verb
1
BAD : If I would live in the countryside, I would be much healthier.
GOOD : If I lived in the countryside, I would be much healthier.
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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 postcard.' 'If I won a lot of money, I'd buy a new car.'
2
BAD : If you would have caught the earlier train, we could have travelled together.
GOOD : If you had caught the earlier train, we could have travelled together.
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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.'
3
BAD : If you would have any more questions, I'll do my best to answer them.
GOOD : If you should have any more questions, I'll do my best to answer them.
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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.'