Modals: deduction & possibility

They must be brothers. They look so similar. = a deduction, we are sure it is true.

She can’t be on a diet. She’s eaten all the cake.= a deduction, we are sure it is untrue.

May, might, could = we are unsure. It’s a possibility.
e.g. He might be his brother, but they are not alike.
She could be French or Spanish, but she’s not Italian.

You could have warned me! = I am annoyed – stress the modal.

I must have been a good mother. All the children turned out OK. = deductions about past situations

It can’t have been easy living without a washing machine!

Why isn’t he at the party? He might have been tired.

She might be a good boss, but she doesn’t know how to manage people.= contrasting ideas
=
Although she’s a good boss, she doesn’t know how to manage people.


I might as well go to the park. There’s nothing else to do.= there is nothing to lose by this action.