MODALS OF OBLIGATION AND PROHIBITION

Obligation
Have to is often used for rules.
You have to work hard. (it's a law) (Dovere, non obbligatorio)
Must is often used when the obligation comes from the speaker. Must is never followed by to.
I must remember it. (I think this) (Obbligo, devo assolutamente ricordarmi)

Prohibition
Mustn't means it is prohibited.