Unit 9 Past modals of deduction

Must/might/can’t have

Must

Use to say that you believe something is a possibility

Use to express that we feel sure that something was true in the past.

Might

Use might to say something is a possibility (but we are not 100% sure).

Can’t have

Use to say that you believe something is not possible in the past.

http://speakspeak.com/english-grammar-exercises/intermediate/modal-verbs-for-past-probability-must-have-cant-have-couldnt-have-may-have