Modals of Deduction
We use modals to make deductions or guesses based on evidence we have in the present.
Must (99% Certain it is true)
Use 'must' when you are almost sure something is true based on logical evidence.
He has been working for 14 hours. He must be exhausted. You've been traveling all day; you must be hungry.
Might / May / Could (50% Certain)
Use these when you think something is possible, but you aren't sure.
I can't find my keys. They might be in the car. She could be at home, but I'm not sure.
Can't (99% Certain it is NOT true)
Use 'can't' (NOT mustn't) when you are almost sure something is impossible.
That can't be John. John is in London right now.