Obligation: Must and Have to

Modals are special auxiliary verbs used to add extra meaning to the main verbs. Each modal shows different meaning in a sentence, for instance, ability, certainty, obligation, permission, and prohibition.

Form: Subject + modal + verb (bare infinitive)

Must and have to are used to show obligation, that is, something you're obliged or strongly advised to do.

Must is stronger and indicate urgency or stress importance. It usually comes from safety instruction or come from one of the speakers; for examples,

• You must fasten your seat belt. (safety instruction)
• I usually forget her birthday. I must remember this year. (from a speaker)
• The baby's asleep. You must be quiet. (from a speaker)

Have to is more commonly used. It comes from a third person, possibly from rules; for examples,

• The government says you have to do military service.
• You have to show some ID when you pay by cheque.