Hi, Shumam!
énoncé 3 : "You must get used to your new position"
He said I had to get used to my new position.
énoncé 4 : "I mustn't use the carriage"
He said I wasn't allowed to use the carriage.
HAD TO :: obligation / contrainte
ALLOWED TO :: permission
Dans la première phrase, on exige que je fasse quelque chose.
Dans la seconde, on permet (ou ne permet pas) que je fasse qqch.
Did that help?