Sono entrambi usati per esprimere obbligo o necessità, ma ci sono delle differenze:
• Must si usa per esprimere le sensazioni della persona che parla, mentre have to esprime un'idea impersonale.
You must clean it. You are obliged to clean it (I require that you clean it)
You have to clean it. You are obliged to clean it. (There's a rule requiring you to clean it)
Must I wear a shirt? Am I obliged to wear a shirt? (What do you think?)
Do I have to wear a shirt? Am I obliged to wear a shirt? (Is there a rule about shirts?)
• Have to esprime principalmente obblighi generali, mentre must viene usato per obblighi specifici:
I have to take the dog out everyday.
I must go to that party.
Importante: Per esprimere obblighi, doveri o necessità nel futuro e nel passato, must e need vengono sostituiti con have to:
We must (need to) buy a new fridge.
We had to buy a new fridge yesterday.
We'll have to buy a new fridge later.