Grammar
Let’s look at ... modals of deduction and speculation (past tense).
When we want to talk about something but we aren’t sure, we can use modals.
Also if we are making conclusions, we can use modals.
Must have = we are sure about something and there is clear evidence. You must have been born before 2000 – I know you are at least 30 years old!
Might have = we aren’t 100% sure about something but we think it might be possible. Janice might have stopped to visit her mum on the way here but I’m not sure.
Could have = this is when we want to say that someone had the ability or the chance to do something but they decided not to do it. You could have spent your gap year in Australia but you went to the US instead.
Should have = this is when you express regret, in other words, you wish you had done something but you didn’t. I should have spent my summer holidays reading books from the booklist for this year.