I have to prepare a class on modals verbs comparing them in English and Spanish. I would appreciate any useful material on theory as well as practice.

A "modal" is a verb that indicates a need or a
possibility. Because of this, modals also identify a "mood" in the verb. In the English
language, modals and modals auxiliaries include : can, may, must, ought,
shall, should, will, and would.

The Spanish version of a modal is
very different because, syntactically speaking, the Romance roots of Spanish do not
directly transfer from the Germanic roots of English. Some linguists even argue whether
the term "modal" could be aptly used as an identifier in Spanish linguistics, since it
seems to be mostly applicable to the morphology of English words. The evidence of this
lays on the fact that only two verbs in the Spanish language, the verbs "poder" and
"deber", are the equivalent of a much longer list of English
modals.

For your presentation, the best thing to do FIRST
is to establish this fact: that Romance and Germanic roots contrast tremendously, and
that Spanish and English belong, respectively, to those two families of language.
Believe it or not, a lot of people take this fact for granted or do not consider it at
all.

Second, build a diagram, or any form of graphic
organizer (I suggest a bubble map or a Frayer Model), where you present the English
modals that are listed at the beginning of this answer.

Third, use a separate graphic organizer (just one showing
two columns divided and use two separate colors *very important* and dedicate one column
and specific color to PODER and another to DEBER.  If you are not using a Smartboard,
use either sticky notes or index cards to place under the correct translation.

For example:

Poder- To be
able to: can, may, will

Deber: To be able to: should,
would, must, shall, ought

It gets a bit confusing as you
conjugate these verbs. For the sake of your presentation, unless you have high command
of Spanish, try to stay away from conjugation. The reason is that, while some of these
modals are repetitive in English, they transform a lot in Spanish.

An example using the word "deber", goes as
follows:

I should- yo debo,

I
should have: yo debi de/ debi haber

I should be: deberia
ser

I should have been: debi haber
sido

So, as you can see, it is best to stick to the history
of the languges and the rules of syntax. Good luck!