The room must be cleaned (by me).解释，谢谢clean的被动语态是"be cleaned".保留must的话，就成了“must be cleaned".I和room一个subject,一个object对调位置，所以the room要放在最前面。by me这一部分则是可有可不有。The room must be cleaned已经带了”by me“的意思了，加上了by me更完整些。