Must may indicate either obligation, or a
deduction/certainty.
Which of
these two meanings does must have in each of these sentences?
You must give me your final answer before the
bell rings.
Let me guess.
It must be a political figure of some sort.
It must be a woman as men don't marry their brothers!!
It must be Cleopatra.
In the first example, must indicates OBLIGATION;
in the others, it indicates a DEDUCTION.
MUST EXPRESSING "OBLIGATION"
The negative is mustn't and the past is
had to:
You mustn't reveal the answer until the
guest has tried to work it out.
Last week he had to give them the answer because they
couldn't guess it.
MUST EXPRESSING "DEDUCTION /
CERTAINTY"
The negative is can't and the past is must have+past participle:
I'm sure it's a political figure. So it
can't be Madonna or
Monica Seles.
It can't be Elizabeth I, because she never married.
Cleopatra must have ruled her country bravely; and she can't have been afraid of dying.
This deductive use of must and can't
is often difficult for Spanish-speaking learners to learn to use.
For this reason, it is practised separately from other modal verb
uses. In Unit 7 of this First Certificate course, this grammar
will be studied more widely.
