if引导的虚拟条件句，
1. 过去的时间，谓语动词是：had + 过去分词
2. 将来的时间，谓语动词是：
be用were ；were to do something；实义动词用过去式
3. 现在的时间，谓语动词be用were； 实义动词用过去式
1. If you had come here earlier, you would have met him.
2.If it were to rain tomorrow, we would not go to the countryside.
3. If you were here now, you would have a talk with him face to face.
