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: ' t" d% N- h% J% B' L Do they have to pay their bill before the end of the month? 0 @9 a2 |3 p: B" g9 d) q S She didn't use to play tennis before she was fourteen. ' @, K7 {( O" k2 g You ought not to have told her all about it. " v% B& E, V9 e. k' d Ought he to see a heart specialist at once.? ( x* r4 I" j/ M& p- s 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。4 W6 T' f1 V$ p; V+ m
典型例题5 U5 M. f$ l( c. a0 H
Tom ought not to ___ me your secret, but he meant no harm. 8 V8 T2 ?4 s+ \, t" _& b) _( L% `9 u A. have told B. tell C. be telling D. having told/ S Z) A" K4 R( T
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.