MODALS VERBS:

We found a web page called "free English lesson". It's in Google, in the first option. The page has a interesting information about these verbs and some examples too. If you don't understand something, it doesn't matter, because it's in Spanish and you must understand it. In this page there are grammar or vocabulary, so it isn't only about modals verbs.
We have learned how to use them. There are ten modals verbs:
can (poder)
could (podría)
may (puede)
might (podría)
 shall (auxiliar)
should (deber)
will (auxiliar)
would (auxiliar)
must (deber obligado)
ought to (debes de)
You can't use them without a principal verb and modal verbs are auxiliary verbs. Ought is the only modal which have to.

If you want to learn about it, you would go to this page.
http://www.elearnenglishlanguage.com/ingles/gramatica/verbosmodales.html