Resumen.

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)

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1.2. Expresar una generalización.

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      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).

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      My sister sometimes has to go to school in the afternoon. (It's part of her timetable).

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      John doesn't have to work on the weekened.

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      Susan: What do you have to do tomorrow?

      Roger: I have to work all day. Why?

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