All the verbs we have seen until now were main verbs, they describe an action. The verb 'to speak' for instance for instance is a main verb. About someone who speaks we know that he doesn't open his mouth to respire like a fish, but to inform us about the results of the activity of his brain. Modal verbs don't describe an action, but the circumstances in which an action is executed.
He has to read the book.
=> He has no alternative, because there is an institution powerfull enough to constrain him.
He want to read the book.
=> Nobody compels him to read the book, he wants to read the book.
It is obvious that the circumstances in which the action is executed are different in these sentences. There are differences in the use of the modal verbs between English and Italian, but for the moment we are not interested in analysing them, we are going to do that later. For the moment we only want to know, how to conjugate them. All modal verbs are very irregular in Italian.
potere => can
volere => want
dovere => must
Io devo leggere il libro.
I have to read the book.
Tu puoi scrivere la lettera.
You can write the letter.
Lui vuole bere una birra. He wants to drink a beer.