Modal verbs, followed by an infinitive, express the speaker’s attitude towards an action, indicating obligation, desire, ability, capability, permission, possibility, probability. In Italian, four verbs can be used as modal verbs: dovere, potere, volere, sapere.
Dovere modifies the verbs to express obligation, recommendation or probability.
Sbrighiamoci … dobbiamo prendere il treno. Hurry up … we must take the train.
Devi comprare frutta e verdura. (You) have to buy fruits and vegetables.
Devi studiare di più. You should study more.
Stanno bussando: dovrebbe essere Francesco. Somebody is knocking: it must be Francesco.
Potere modifies the verbs to express possibility, permission and also ability together with willing. In conditional tense, it is useful to ask someone to do something in a polite way.
Posso uscire? Can I go outside?
Posso portare io fuori il cane, se non ti va. I can take the dog out, if you don’t want to.
Puoi chiudere la finestra? Can you close the window?
Potresti chiudere la finestra? Could you close the window?
Volere modifies the verbs to express desire. In conditional tense, it expresses the desire in a polite way.
Voglio andare al parco. I want to go to the park.
Vorrei andare al mare. I’d like to go to the sea.
Sapere modifies the verbs to express ability: simply being able to do something.
Vivi in un posto di mare e non sai nuotare? You live at the beach and you can’t swim?