"Must" is internal obligation or logical deduction; "have to" is external obligation imposed by others.
Obligation coming from the speaker themselves, or logical deduction about something. Emotionally stronger.
I must call my mother. I have not spoken to her in two weeks.
Debo llamar a mi madre. No he hablado con ella en dos semanas.
You must be exhausted after that long trip.
Debes estar agotado despues de ese largo viaje.
This restaurant must be good — there is always a line outside.
Este restaurante debe ser bueno. Siempre hay fila afuera.
Obligation imposed by rules, laws, another person, or circumstances. Neutral and factual.
You have to wear a helmet when riding a motorcycle. It is the law.
Tienes que usar casco cuando montas motocicleta. Es la ley.
She has to work on Saturdays this month. Her boss asked her to.
Ella tiene que trabajar los sabados este mes. Su jefe se lo pidio.
Do we have to pay in advance, or can we pay at the door?
¿Tenemos que pagar por adelantado, o podemos pagar en la entrada?
Common mistake
Using "must" for external rules: "You must pay taxes" works but sounds like the speaker is personally imposing it. "You have to pay taxes" = it is the law, an external obligation. Critical error: in negatives, "must not" and "do not have to" are OPPOSITES. "You must not smoke" = it is forbidden. "You do not have to smoke" = it is not required (you can choose not to).
Trick to remember it
The most important rule of this pair: MUST NOT = prohibition (you cannot). DO NOT HAVE TO = not required (you can choose). This difference is critical. For positive obligation: must = I decide it is necessary. Have to = someone else or the rules impose it.
Quick rule
Must = internal obligation / deduction. Have to = external obligation. Must not = forbidden. Do not have to = not required.
Did you know?
"Must" has no past tense in modern English. For the past, you use "had to": "I had to work late yesterday". "I musted work late" does not exist — ever. This is one reason "have to" is more versatile: it has all verb forms (past, future, continuous) that "must" cannot express.
Exercise 1 · Pick the right one
You _____ be kidding me. That price is outrageous.
Exercise 2 · Pick the right one
You _____ return the library books by Friday. There is a fine after that.
Exercise 3 · Form the word
Must = internal obligation / deduction. Have to = external obligation. Must not = forbidden. Do not have to = not required.
Using "must" for external rules: "You must pay taxes" works but sounds like the speaker is personally imposing it. "You have to pay taxes" = it is the law, an external obligation. Critical error: in negatives, "must not" and "do not have to" are OPPOSITES. "You must not smoke" = it is forbidden. "You do not have to smoke" = it is not required (you can choose not to).
The most important rule of this pair: MUST NOT = prohibition (you cannot). DO NOT HAVE TO = not required (you can choose). This difference is critical. For positive obligation: must = I decide it is necessary. Have to = someone else or the rules impose it.
Bryan López
Founder of SpeakUP Academy