How do we know that the Blessed Mother or Pharoah had free will?

Whydid God harden Pharoh’s heart in the book of Exodus? Does this contradict the gift of God’s gift of free will? My high school seniors insist that not all men have free will. They insist that God used certain people (pharoh, Judas, and others I can’t recall) to carry out his plan; therefore these people had no free will. God is this cruel being who creates some humans to be used and be damned. I’d like to be able to guide my students on the right path. Can you please refresh my memory about what it really meant to “harden his heart”. Also, reading the annunciation, they claim that Mary also did not have free will when having to bear a child. They claim that the angel came to give her a command, and she had no choice but to accept. “Behold, you will conceive in your womb and bear a son, and you shall name him Jesus.” Any thing you can share, I am sure will help.
Thank you for taking the time to reply.

Lisa

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Recommended Reading:
Did Mary have a choice?

Predistination vs. Free Will
Do Catholics believe in predestination?
Is Calvinism compatible with Catholicism?
Why would God create those he knows will go to hell?

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