Identity Sentences
Like its English equivalent to be, the Spanish verb ser can be used either to indicate the relation of identity or as a copula. The difference is apparent in the following pair of examples:
(1) Pedro es el médico.
(2) Pedro es (un) médico.
The es of (1) is a relational term indicating that the flanking expressions designate one and the same object. Using the identity sign =, the broad logical structure of (1) can be represented as a = el F, where a is Pedro and Fs are doctors. On the other hand, sentence (2) classifies Pedro as a doctor and so the structure is just Fa.