If it should rain tomorrow,we will not do
谓语动词的虚拟形式
从句\x05 主句
现在 动词过去式（be 多用were）\x05 would / should / could / might + 动词原形
过去\x05had + 过去分词\x05 would / should / could / might + have + 过去分词
将来 动词过去式或 should / were to + 动词原形\x05would / should / could / might + 动词原形