Must have

Bonjour Jennifer, et merci du compliment !

To express "must have", you will conjugate the verb "devoir" in Le Passé Composé followed by your verb's infinitive:
"Elle a dû aller faire les courses." (She must have gone to the shops.)
"J'ai dû me tromper." (I must have made a mistake.)

Here's a link to Le Passé Composé of "devoir":
https://french.kwiziq.com/my-languages/french/view/4612

I hope that's helpful!
À bientôt !

Ought to have,

Mercii Aurélie.

Would you also help me out with ought to? what would we say for example for he/ought/should have done their homework. I ask this because I now realise that the difference between ought, should and must have are becoming difficult for me to destinguish.

Thank you once again

"Ought to" and "should" will both be translated in the same way in French, as they mean the same:
with "devoir" in Le Conditionnel.

:)

Thank you