对过去假设：从句用过去完成时,主句用would／should／could／might+have+过去分词.对现在假设：从句用过去时（be动词用were）,主句用would／should／could／might+动词原型.对将来假设：从句用过去时／were+to do／should,主句用would／should／could／might+动词原型.