Hi John,

This is quite subtle and it is when 'must' equals an imperative need to do something, rather than an obligation as the following examples will illustrate:

Je dois aller chez ma mère ce matin, elle ne va pas bien.

Je dois aller aux toilettes.

Je dois passer à la banque, je n'ai plus d'argent.

You would use 'devoir' rather than 'avoir besoin de' and as you have noticed it is followed by another verb in the infinitive.

Hope this helps!