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: + n/ T5 m, D' e; C Do they have to pay their bill before the end of the month?2 E. ^( d& q1 T/ O# ?& N
She didn't use to play tennis before she was fourteen.! E" K' n: | G
You ought not to have told her all about it. , B+ M# ?7 m1 R- |) A Ought he to see a heart specialist at once.?6 O# m. N: m, {2 r' A+ e5 x
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。9 r3 t% `' r% @. T5 X
典型例题4 c( W- b" A' R
Tom ought not to ___ me your secret, but he meant no harm.$ ?4 ^' l9 H* Z
A. have told B. tell C. be telling D. having told 5 q* U! y v; l 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 told.