Grammar. Must and have to.
1. Usamos must para:
1.1. Expresar una obligación interna. Algo que uno se impone realizar.
I must wash my car. (You see it's dirty)
1.2. Expresar una generalización.
I think we must study hard for the finals.
2. Usamos have/has to para expresar una obligación externa. Algo que no podemos evitar cumplir. Ejemplo: La ley, los contratos de trabajo, obligaciones legales contraídas con terceros, etc.
I have to work from Monday to Friday. (My contract says so).
My sister sometimes has to go to school in the afternoon. (It's part of her timetable).
John doesn't have to work on the weekened.
Susan: What do you have to do tomorrow?
Roger: I have to work all day. Why?