You must be mistaken.
你一定搞错了。
must的意思是“必须”,比如下命令时老板有时会很强硬地说,
"You must finish this book before this weekend."
(你必须在周末前完成这本书。)
在下面的这句话里must的意思有些变化,有了很强的推测语气,
"He must have passed the exam, because he was so happy the whole day."
(他一定已经通过考试了,因为他整天都那么高兴。)