I must - debo / debo de

Affirmative

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• I must go to the library.
  Debo ir a la biblioteca.
• You must stay here until I come back.
  Debes quedarte aquí hasta que yo vuelva.
• We must be at the airport at three o'clock.
  Debemos estar en el aeropuerto a las tres.
• Everybody must wear a uniform.
  Todos deben usar uniforme.
• You must be joking!
  ¡Deben de estar bromeando!
• Julia must be in Paris by now.
  Julia ya debe de estar en París.
• It must be noon already.
  Ya debe de ser mediodía.

Negative

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• You must not talk to strangers.
  No debes hablar con extraños.
• We mustn't make noise.
  No debemos hacer ruido.
• Mr. Williams must not smoke.
  El señor Williams no debe fumar.

Interrogative

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• Must we do everything today?
  ¿Debemos hacer todo hoy?
• Must you go so soon?
  ¿Debes irte tan pronto?

Must + have

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• I must have fallen asleep.
  Debo de haberme quedado dormido.
• Sally must have forgotten about the meeting.
  Sally debe de haberse olvidado de la reunión.
• They must have sent it already.
  Ya deben de haberlo enviado.