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.)