must不用在虚拟语气中
虚拟条件句：
条件状语从句是非真实情况，在这种情况下要用虚拟语气。
1．条件从句与现在事实不一致，其句型为：
句型1：If ＋主语＋过去时，主语＋ should (could， would， 或might) ＋动词原形
If I were you， I would study hard.
If it rained， I would not be here now.
2．条件从句与过去事实不一致，其句型为：
句型2 : If ＋主语＋had＋过去分词，主语＋ should（could， would， 或might）＋ have ＋过去分词
If the doctor had come last night， the boy would have been saved.
If I had not studied hard， I would have failed in the exam last term.
3．条件从句与将来事实不一致，其句型为：
句型3：If ＋主语＋过去时/ should ＋动词原形/ were to＋动词原形，主语＋ should (could， would， 或might) ＋动词原形
If it should rain tomorrow， we would stay at home.
If I were to go to the moon one day， I could see it with my own eyes.
If you missed the film tonight， you would feel sorry.
