The helper or modal verbs potere (to be able to, can), volere (to want), and dovere (to have to, must) take on different meanings in different tenses.
Potere, for example, can mean “to be able to,” “can,” “to succeed,” “could,” or “would be able to,” depending on the context and tense. Dovere can mean “to owe,” “to have to,” “must,” or “to be supposed to,” according to the tense.
The Italian modal verbs precede the infinitive of another verb, and indicate a mode (respectively: necessity, possibility, volition):
Non ho potuto aiutarlo (possibilità)
Rita vuole dormire (volontà).
To underscore the close link between the modal verb and the verb that follows it, the former usually takes the auxiliary of the second:
Sono tornato / Sono dovuto (potuto, voluto) tornare;
Ho aiutato / Ho potuto (dovuto, voluto) aiutare.
But it is common to encounter modal verbs with the auxiliary avere, even when the governing verb requires the auxiliary essere:
Sono tornato / Ho dovuto (potuto, voluto) tornare.
In particular, the modal verbs take the auxiliary verb avere when they are followed by the verb essere:
Ho dovuto (potuto, voluto) essere magnanimo.
POTERE
In the present indicative tense, potere means “to be able to” or “can.”
Posso uscire? (May I go out?)
Posso suonare il trombone. I can (am able to) play the trombone.
In the present perfect tense, potere means “to be able to, to succeed”:
Ho potuto spedire il pacco.
(I was able to mail the package.)
Non sono potuti venire più presto. (They could not come earlier, but they tried.)
In the conditional tenses (condizionale presente and condizionale passato), this verb may be translated as “could,” “would be able to,” “could have,” or “could have been able to”:
Potrei arrivare alle tre.
(I was able to arrive at three o’clock; I would be able to arrive at three o’clock.)
Avrei potuto farlo facilmente. (I could have done it easily; I would have been able to do it easily.)
VOLERE
In the present indicative, volere means “want.”
Voglio quell’automobile. (I want that car.)
In the present perfect (conversational past), volere is used in the sense of “decided, refused to”:
Ho voluto farlo. (I wanted to do it; I decided to do it.)
Marco non ha voluto finirlo. (Mark didn’t want to do it; Mark refused to do it.)
In the conditional, volere means “would like”:
Vorrei un bicchiere di latte. (I would like a glass of milk.)
Vorrei visitare i nonni. (I would like to visit my grandparents.)
DOVERE
The present indicative forms of dovere translate as “owe.”
Gli devo la mia gratitudine. (I owe him my gratitude.)
Ti devo venti dollari. (I owe you twenty dollars.)
In the conditional tenses, however, dovere carries the meaning “should” or “ought to.” For example:
Dovrei fare a tempo a finire i compiti di scuola. (I should/ought to finish my homework on time.)
Avrei dovuto telefonarle immediatamente. (I should have/ought to have telephoned her immediately.)