Expression de l'obligation et de l'absence d'obligation

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.)