Not only Mary but also her parents (have) seen the film already.
句意：不但玛丽已经看过这部电影,而且她的父母也看过了.
理由：时态是现在完成时,要用助动词have \ has.not only...but also使用的是就近原则,由离动词谓语最近的主语来确定单、复数,因为最近的主语是parents,所以要用复数形式have.