托福语法辅导：带to的情态动词

会计考友

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：
2 e- y6 F& b# r+ e9 t    Do they have to pay their bill before the end of the month？
/ J/ z1 g0 V3 l( W4 G    She didn't use to play tennis before she was fourteen.
# I7 P) J8 k& x2 O    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。
    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，此地应用过去完成时，但它在情态动词 ought to 后，所以用have told.