Copular constructions have raised a number of important challenges for linguists, philosophers, and developmental psychologists alike, particularly in the domain of to be clauses in Spanish. In contrast to English, Spanish uses two distinct forms of to be: ser and estar. Using Mexican Spanish as the language of study, this dissertation proposes a reanalysis of classic assumptions about ser and estar as copular verbs and argues instead that ser is the only copula in Mexican Spanish while estar is a neo-Davidsonian stative verb, hypotheses that until now had not been considered. The reexamination of ser and estar further brings to light the current status of semantic change in estar as used with adjectives, differentiating Mexican Spanish from other dialects of Spanish. I demonstrate experimentally that estar's use is being extended innovately in adjectival contexts to contexts otherwise used with ser. This novel experimental finding serves as a backdrop for investigating the early acquisition of ser and estar in a dialect otherwise ignored. Experiments focusing on adjectival predicative expressions demonstrate a two-stage developmental pattern in young children whereby semantic sensitivity to the verb's basic differing semantics is observed during Stage 2 beyond the age of 3;6 years old.