句子结构为：从句：If + 主语+ should (或were) + 动词原形+……主句：主语+ would (could,should,might) + 动词原形+……
1.If it should rain tomorrow,I would stay at home.如果明天下雨的话,我将待在家里.（根据天气情况,明天不可能下雨）
2.If he came tomorrow,I would do it with him.如果明天他来的话,我将和他一起做此事.（事实上他来的可能性很小）
3.If he were here,I would give him the books.如果他在这儿,我可能会把书给他.