这个用户的提问，其实就是你要的答案

http://ask.yygrammar.com/q-42584.html

-I spent a week in Xiamen last month. -Then you must have been overcharged for sea food many a time, _____ you? 这道题填weren't,为啥不能填haven't或是didn't? 这里有many a time不应该用现在完成时haven't吗？didn't只能用在must have done表一般过去时的非被动语态里吗？ She must have been there yesterday, didn't she? 这里可以用wasn't she吗？wasn't/weren't只能用在must have been done被动句里吗？

    【答】这道题不能填weren't, 应该填haven't或是didn't。

     北京语言大学出版社2012年7月答1版《新编英语句型句法全解》在 22.2.4.4 附加疑问句中简略问句的动词 中指出：“4）当陈述部分的情态动词表示推测时，那么简略问句要用第二个动词的限定性形式；在表示对过去情况推测时，简略问句的动词可以用第二个动词的限定式，也可以用的限定式。如：...... You must have read the book, haven't you / didn't you?”

      ”must have + 过去分词”是表示对过去情况的推测。所以：

      -I spent a week in Xiamen last month. 

      -Then you must have been overcharged for sea food many a time, haven't you / didn't you?”? 

即使按照国内语法书的解释，陈述部分含有表示推测的情态动词，问句部分不用情态动词，那么这个句子的语境中显然有一个确切的过去时间状语last month. 所以问句部分用现在完成时为语法错误。只能用一般过去时 weren't 。