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 (shall), tener 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.