The authorities must do something about the traffic congestion.
John’s not here yet. He must be stuck in traffic.
To understand the difference in meaning of these two examples it’s best to look at modal verbs using the words intrinsic and extrinsic which are often used to describe modal verbs.
Intrinsic meanings are meanings which give the idea that people have some control over the actions or events. These meanings can be put into groups like permission, necessity and willingness.
Extrinsic meanings are meanings where people express their ideas or thoughts about what is or isn’t going to happen. These meanings can be put into groups like possibility or prediction.
So, ‘The authorities must do something about the traffic congestion’. The meaning of ‘must’ is intrinsic and describes a necessity.
And ‘John’s not here yet. He must be stuck in traffic.’ The meaning of ‘must’ here is extrinsic and describes certainty or a real possibility.
The verbs ‘may’ and ‘might’ are used to say that something is possibly true:
I think I might be coming down with the flu.
That may be a better solution.
In informal situations ‘might’ is more common than ‘may’ when we want to talk about what someone will do in the future:
They might move to London.
I might go out tonight.
‘May’ is more commonly used in formal situations:
The price of the property may go up in the future.
‘May’ is not usually used in questions asking about how true something could possibly be:
How possible is it for you to come first? Are you likely to win? And not “May you win?”
‘May’ and ‘might’ – permission
Usually ‘may’ is used to talk about permission in formal situations:
Laptops and tablets may not be used during the test.
Only hotel guests may use the facilities.
‘May’ is used for formal requests:
May I ask a question?
May I come in?
Part 2 - Modals B – must/have to (Necessity – Deduction)
Part 3 - Modals C – should/ought to (Obligation and Probability)
Lesson by Tristan, English teacher at EC Malta English school
Decide if these sentences denote Permission, Possibility
