线话英语语法课堂：带to 的情态动词

,英语书面语

　　 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。它们的疑难，否定形式应予以注意：
Do they have to pay their bill before the end of the month?
She didn't use to play tennis before she was fourteen.
You ought not to have told her all about it.
Ought he to see a heart specialist at once.?

　　　 ought to 本身作为情态动词利用。其余的词作为实义动词应用，变疑难，否认时，须有do 等助动词辅助。 CopyRight .com

　　典型例题
Tom ought not to ___ me your secret, but he meant no harm.　
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，此时用过去完成时，但它在情态动词 ought to 后，所以用have。