疑问词作主语,谓语动词用第三人称单数形式。能作主语的疑问词一定是疑问代词,因疑问副词不能作主语。疑问代词作主语,常把疑问代词看作是第三人称单数形式,根据“主谓一致”原则,谓语动词用第三人称单数形式。例句:What makes you think I am a doctor?Who came here to borrow your book?