I would play football with you if I _____ 20 years younger. (be

 I would play football with you if I _____ 20 years younger. (be)
【正确答案】：were
【题目解析】：if引导的虚拟语气，表示对现在事实的假设，从句用动词的过去式，be动词用were，主句用would/should+动词原形。