帶to 的情態動詞有四個：ought to， have to， used to， be to， 如加上have got to (=must)， be able to，為六個。它們的疑問、否定形式應予以注意：

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 本身作為情態動詞使用。其他的詞作為實義動詞使用，變疑問，否定時，須有do 等助動詞協助。

典型例題

Tom ought not to ___ me your secret， but he meant no harm.

A. have toldB. tellC. be tellingD. having told

答案A.由于后句為過去時，告訴秘密的動作又發生在其前因，此地應用過去完成時，但它在情態動詞 ought to 后，所以用have told.