When you believe something is true, you speculate. To speculate we use modals of deduction such as : might, may, must, could, can’t.

To speculate about the past :

– You must have been so happy when he proposed.
– He may/might/could have been stressed about proposing to you.
– You can’t have been feeling unsure, you love him so much.

Deductions about the present :

– You must be so happy.
– He may/might/could be stressed.
– You can’t feel unsure, you love him so much !