You must be mistaken.
你一定搞錯了。
must的意思是“必須”,比如下命令時老板有時會很強硬地說,
"You must finish this book before this weekend."
(你必須在周末前完成這本書。)
在下面的這句話里must的意思有些變化,有了很強的推測語氣,
"He must have passed the exam, because he was so happy the whole day."
(他一定已經(jīng)通過考試了,因為他整天都那么高興。)