中学英语入门之带to的情态动词

　　带to的情态动词的中学英语入门语法点：

　　  
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，否认形式应予以留心：
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，否定时，须有do 等助动词辅助的中学英语入门解析点。

典型中学英语入门例题
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。由于后句为从前时，告诉秘密的动作又发生在其前因，此天时用从前实现时，但它在情态动词 ought to 后，所以用have。