89
12.4 deductions and speculations about the present must, cannot, should 1.
Must is used for drawing logical conclusions
in the affirmative form have to is not generally used in such contexts.
2.
Cannot is used for drawing logical conclusions in the negative form.
3.
Should indicates what is likely (but not certain) to happen.
YES NO 1 If X = 1 and Y = 2, then X + Yb mustb equal three. If X = 1 and Y = 2, then X + Yb has to equal three.
2 If X = 1 and Y = 2, then X + Yb cannotb equal five. If X = 1 and Y = 2, then X + Yb must not equal five If the two
substances are mixed together, they
should gored. However, occasionally the mixture is brown. If the two substances are mixed together they
must gored. However, occasionally the mixture is brown.
90
12.5 deductions and speculations could, might (not) 1.
Could is often used to suggest a possible course of action.
2.
Might indicates a possible reaction to or consequence of a course of action – but there is no certainty that this reaction or consequence will take place.
3.
The difference between could and
might is occasionally very subtle –
could has the sense of certainty,
might of uncertainty (this mayor may not happen.
4. Sometimes
could and
might can be used interchangeably.
5.
Could not is
not used to make speculations, instead
cannot is used
might not means that there is a possibility that something is not true.
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