Il existe plusieurs façons d'exprimer l'obligation :

  • Au présent
    Must + base verbale "You must be quiet!"
    Have to + base verbale "I have to leave now."
    Have got to + base verbale "they've got to move their car."
     
  • Au futur et au passé
    On emploie : will have to / had to.
    "You 'll have to move if you get this job."
    "They had to borrow money to pay for their house."

  • Attention ! "Mustn't" exprime l'interdiction et non l'absence d'obligation :
    "You mustn't make noise" = tu ne dois pas faire de bruit. (C'est interdit.)

 

Pour exprimer l'absence d'obligation (ce qu'on n'est pas obligé de faire), on utilise :

  • Au présent :
    Don't have to + base verbale "You don't have to rush. You've got plenty of time."
    Needn't + base verbale "You needn't rush. You've got plenty of time."
  • Au futur :
    Won't have to + base verbale "You won't have to get up early, we are leaving at 10."
  • Au passé :
    Didn't have to + base verbale "I didn't have to pay for the entrance. It was free."
  • Attention : needn't have + participe passé = tu n'aurais pas dû.
    "You neend't have brought me flowers " (= tu m'as apporté des fleurs, mais ce n'était pas nécessaire.)