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Modal verbs – speculation & deduction

Level: B2
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One of the functions of modal verbs is to express speculation.
Depending on the degree of probability of our deduction, we use different verbs.


  •  The verb “must” is an almost absolute certainty:

The phone’s ringing. It must be Paul.

  • Its opposite is “can’t” and “couldn’t”, meaning that a situation does not happen.

The phone’s ringing, but it can’t be Paul. He’s still at work.

  • The verbs “may/might/could” are used to express possibilities, but could is a little weaker in its expression:

The phone’s ringing. It may be Paul.
The phone’s ringing. It might be Paul.
The phone’s ringing. It could be Paul. 

probability verb example
0% Simple Present
(negative sentence)
He doesn’t know.
unlikely can’t/ couldn’t He can’t know.
50% probability could He could know.
quite likely may/ might He may know.
very likely must He must know.
100% probability Simple Present He knows.
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