Devoir + infinitive sometimes expresses an obligation :
Je dois remporter cette compétition ! > I have to win this contest.Je dois rentrer chez moi avant minuit. > I have to go back home before midnight.Devoir + infinitive sometimes expresses a probability :
Il doit être dans l’avion à l’heure qu’il est. > He must be in the plane by now.Il doit faire froid en hiver dans cette région. > It must feel cold during winter in this region.With the infinitive, negation words are placed before the verb.
Ne pas devoir parler. > Not having to speakNe jamais devoir mentir > Not having to lie.If *devoir* is conjugated, they are placed around it.
Je ne dois pas manger trop gras. > I mustn't eat to fat.Ce film ne dois pas être génial. > This movie mustn't be great.Adverbs are often placed between devoir and the infinitive.
Tu dois absolument lire ce bouquin ! > You absolutely have to read this book!On doit vraiment pas être au bon endroit… > We must really not be in the right place…