My father must be in his office,isn't he?
must 的反意疑问句规则较多,本句中must表示猜测“一定,肯定”,反意疑问句的附加谓语不用must和need,而是在意义上和陈述句中谓语动词的时态保持一致,即must之后的动词be.类似的例如：He must be from the south,isn‘t he?他肯定是南方人,是不是?
补充的问题：
the book is mine 同义可改为：
the book belongs to me.