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GRAMMAR REFERENCE

Modal auxiliaries used for logical deductions; must; might; might not; can't

Compare the following pairs of sentences:

I. a) That car is expensive; b) That car must be expensive

a) I know for a fact it is. b) I'm sure it is because of its appearance.

II. a) He isn't in. b) He can't be in.
a) I know for fact he isn't, b) I'm sure he isn't; maybe he isn't answering
his phone.

Sentences of this kind using must and can't, are the result of the logical thinking, not
expressions of the known fact. They are not always, however, less sure or less true
than factual statements.

Example: Someone commits suicide and we say, He can 't have been very happy.
(This is said not because we are less than 100% sure ( it is 100% certain that the
person wasn't happy), but because we didn't know him, and so have to rely on logical
reasoning.
The person committed suicide. People who commit suicide aren't happy. Therefore:
He can 't have been happy.
We use must when we are sure something is so.
We use can't when we are sure it isn't so.
When we are not sure, we use might.
Talking of the past we use must/ can 't/ might + have done rather than do.
Talking about an action in progress we use the Present Participle of the verb after be:
must/ can't/ might be + doing (Present) or have been: must/ can 't/ might have + been
doing (Past).

Exercises:
Read the following pairs of sentences and define the difference between them.

I. a) They might be having a meeting. b) They must be having a meeting.
II. a) He can't have made such mistake. b) He might have made such mistake.
III. a) You must be tired. b) You must have been tired,
IV. a) They must be very well-off. b) They can't be very well-off.
V. a) They were going too fast. b) They must have been going too fast.
VI. a) She might have been telling b) She can't have been telling the truth.
the truth.

Statement made with must and can't are often backed up by conditional sentences.
Might is often used to suggest other possibilities.

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