To get the basics about why and how we use modal verbs in Italian, and how they are conjugated, see Daniela's video lesson about modal verbs. The modal verbs are: dovere (to have to, must), potere (to be able to, can), and volere (to want to, would).
Italian modal verbs have some similarities with English modal verbs, because they are used together with verbs in the infinitive, but there are differences, too. In English, for example, we can use "to be able to," which does get conjugated, or "can," which doesn't get conjugated. Italian modal verbs are conjugated and are irregular, so as Daniela says, you just have to learn them. These verbs are used so often that you're bound to learn the principle conjugations just by listening. Here's a quick conjugation chart for the present tense, plus a few tips.
There are other verbs like sapere (when it means "to be able to") that are also considered to be modal.
Non lo so spiegare.
I can't explain it [I don't know how to explain it].
When in the regular present tense, using modal verbs is mostly trouble free, as long as you've learned the irregular conjugation. The easy part (handy for when you're not sure of the conjugation of another verb) is that the other verb is going to be in the infinitive!
Let's look at some practical examples. Look for an infinitive verb in the vicinity of the modal verb, to put the modal picture together.
Zia, che cosa devo fare?
Aunt, what should I do?Play Caption
Alex vuole imparare il tedesco.
Alex wants to learn German.
Caption 22, Corso di italiano con Daniela - DomandePlay Caption
Alle mie spalle, potete vedere la statua del Cristo di Maratea.
Behind me you can see the statue of Christ of Maratea.
Caption 1, Antonio - Maratea, Il Cristo RedentorePlay Caption
Let's remember that the verb in the infinitive might actually be missing from the sentence itself, but it can easily be imagined, just like in English.
One very common way modal verbs are used is with the impersonal. See these lessons about the impersonal, which uses the third person, as in the example below.
Si può aggiungere il caffè, si possono aggiungere tanti ingredienti.
One can add coffee, one can add many ingredients.
Caption 10, Andromeda - in - Storia del gelatoPlay Caption
So far we've been looking at the present tense. A bit further along the line, we'll get into modal verbs with compound tenses, which is a bit more complex. Hope to see you then!