How are they used?
Modal verbs are almost always followed by an infinitive verb.
Posso uscire. = I have the possibility.
Voglio parlare. = I have a wish.
Devo studiare. = I have an obligation or a need.
Devi partire, not “devi parti”.
Short rule: with potere, volere and dovere the second verb stays in the infinitive: parlare, vedere, dormire.

