带to 的情态动词有四个：ought to, have to, used to, be to, 如加上have got to ,(=must), be able to，为六个。它们的疑问，否定形式应予以注意：来源：考试大 Do they have to pay their bill before the end of the month? She didn t use to play tennis before she was fourteen.来源：考试大 You ought not to have told her all about it. Ought he to see a heart specialist at once.?
ought to 本身作为情态动词使用。其他的词作为实义动词使用，变疑问，否定时，须有do 等助动词协助。
典型例题 Tom ought not to ___ me your secret, but he meant no harm. 来源：考试大 A.have told B.tell C.be telling D. having told 答案A。由于后句为过去时，告诉秘密的动作又发生在其前因，此地应用过去完成时，但它在情态动词 ought to 后，所以用have。