Pouvoir is can, devoir is must or have to, and each is followed by a bare infinitive with no preposition in between. Il faut is the impersonal must: it never changes for the person, so il faut partir covers "I/you/we have to leave" and the context supplies who.
Examples
- Tu peux me rappeler plus tard ?
- Can you call me back later?
- Je dois envoyer un message maintenant.
- I have to send a message now.
- Il faut répondre avant ce soir.
- We have to reply before this evening.
The mistake to avoid
Nothing comes between the modal and the infinitive - say "je dois envoyer", never "je dois d'envoyer".
Where this fits in
Téléphone, messages et petits problèmes, French A2: Appeler et écrire, s'excuser, dissiper un malentendu et demander de l'aide : le quotidien quand quelque chose ne se passe pas comme prévu.