Modal verbs modify a full verb without having a lexical
meaning. Now first what is a full verb? A full verb
is verb with a lexical meaning, like for instance to swim.
If this verb is used we know two things: somebody is
in a liquid, probably water and the person or thing
does not sink. The full verb informs us about the activity
of a subject.
Modal verbs as 'to have to, may,
can, shall' do not inform us about any
real activity, yet. The change the meaning of the full
verb or better they clarify the relationship between
the subject and its activity.
He can swim.
He must swim. He shall swim. He may swim.
As we can see in the examples the relationship between
the subject (he) and the activity (to swim) is different
in every of the sentences.
Spanish modal verbs like querer
(to want to), deber
que (to have to),
poder (to be able to) were already used in this manual. Now we
should have a better look at the use of the modal verbs.