One use for must (negative
must not or mustn't)
is in showing
Another use is in making conclusions--making guesses which
based on strong, convincing evidence. These guesses may
be either affirmative ( + ) or negative ( - ).
Akihiro got a very high score on the TOEFL.
He must know English
conclude that Akihiro knows English well
I know that the TOEFL is very difficult
also know that Akihiro got a very high score.)
I'm not sure
what time it is, but judging by the sun,
it must be around 2:00 PM.
(I conclude that the time
is around 2:00 PM because
I know the approximate
times for different positions
of the sun.)
no lights and no one is answering the doorbell.
must not be at home.
(Because there are no lights
and because no one is
answering the doorbell,
I conclude that no one is at home.)
When I said,
"Could you help me?," that woman had
blank look and said something in another language.
must not know English.
(I conclude that the woman
doesn't know English
because she had a blank look
and because she said
something in another language.)
For negative requirements,
both must not and
commonly used, but for negative
must not is commonly used:
You must not
do that! / You mustn't
You must not
be late! / You mustn't
He didn't eat his pie. He
must not like it.
She looks puzzled. She must not know the answer.
Negative requirements and
have different stress:
You must nót live here! (negative requirement)
múst nòt live here. (negative conclusion)
You must nót be tired! (negative requirement)
múst nòt be tired. (negative conclusion)