Are there modal verbs in Spanish? How can modality be expressed in Spanish?

In Spanish there are no proper modal verbs, but we use what we call 'perífrasis verbales modales'. These are constructions involving an auxiliary verb conjugated and a main verb in a non-conjugated form (in modal constructions, an infinitive). The auxiliary conjugated verb conveys information about modality, while the non-conjugated infinitive verb conveys information about the action or its content. Often there are prepositions or conjunctions linking the auxiliary and main verb: these are parts of fixed structures and lose their original meaning as propositions. The main constructions to be used are:
NECESSITY OR OBLIGATION: deber+infinitive, haber de+infinitive, tener que+infinitive, hay que+infinitive
e.g.: deberíamos ir al parque; teníamos que hacer la compra
CAPACITY: poder+infinitive
e.g.: no puedo volar
POSSIBILITY OR PROBABILITY: poder+infinitive, deber de+infinitive
e.g.: el PSOE puede ganar las elecciones, debe de estar con sus amigos
APPROXIMATION OR SUPPOSITION: deber de+infinitive, venir a+infinitive
e.g.: esta bolsa viene a pesar 10 kg
INTENTION: ir a+infinitive
e.g.: voy a estudiar todos los días
One very common mistake is to confuse deber+infinitive with deber de+ infinitive. Note the former entails necessity or obligation, while the latter entails possibility or supposition. In the sentence 'Deben de ser las 3', one is supposing or guessing it is 3. In such content 'Deben ser las 3' is wrong: that would be commanding or obligating time to be 3. Conversely, in 'debes hacer la compra' one is commanding someone to do the shopping. To express this 'debes de hacer la compra' is wrong.