We use modal verbs in a situation where we need some level of deduction which means we say how sure we are about something.
We use 'must' when we feel sure that something is true because we have some information about the situation, we have strong evidence.
She must live close to where she works because she walks to work. (the speaker doesn’t know where but is sure it is not far away)
You’ve been working in the garden all day. You must be tired.
Being a sky diving instructor must be very exciting.
(Must+infinitive without 'to')
Might, may, could
We use 'might', 'may' and 'could' when we are not sure about something.
I don't like the sound of the engine. It could be a mechanical problem.
We might need to work through the night if we want to get this ready on time.
He may not like that shirt. I don’t think it's his style.
(might/may/could+infinitive without 'to')
We use 'can't' to show that we think something is not true.
It can’t be a mechanical problem. I've just taken the car to a mechanic.
They can't be very happy about working during the night.
(can't+infinitive without 'to')
'Must', 'might', 'may', 'could' and 'can't' have many more meanings and are used to talk about different situations. Here we have looked at them used for a present deduction.
Lesson by Tristan, teacher at EC Malta English school
Now put the right modal for these situations: