A.  肯定句    100%       否定句

must         |       can’t, couldn’t

have (got) to*  |        must not

may         |        may not

might, could   |       might not

0%

         Many professors require that students type their schoolwork in college.

(肯定的推測:) The computer must be essential for many students.

        The market for computer sales has got to be increasing for the past 5 years.

(較不肯定的推測:) College students may own a PC of their own.

                They may not be able to afford to buy one if it’s too expensive.

*have to, have got to均不用在否定句裡.

B. 問答句

e.g. “Someone’s ringing the bell. Who could it be?

   “It’s already 11p.m. Could she work till this late?”

   “She could /might /may /must/ has to. It’s her new job, and it’s very demanding.”

  Is Haley still with the China News?

   I’m not sure. She might not be. 

（更上一層樓：通常請求不只包括一個問句，還有稱謂Form of address、開頭語Opening, 請求起頭Prerequest，理由Reasons，請求Request本身，詳見Requests 英文解說）