同学你好，该知识点来自沪江网校《英语零基础直达大学六级【超值特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
must表示推测，要用must+be+形容词/现在分词，或者must have done的形式。
You have worked hard all day.You must be tired.你辛苦干一整天，一定累了。
He must be working in his office. 他一定在办公室工作呢。
I didn't hear the phone. I must have been asleep.　我刚才没有听到电话，我想必是睡着了。

must become是“必须变得……”，意思就不对了。
可以改成will certainly become

祝同学学习进步~