The Spanish imperfect subjunctive verb tense has two competing forms –ra and –se which vary in frequency of use according to the country of usage, the dominant political power, the century in question and context. These two forms of the imperfect subjunctive have their origin in two different verb tenses in Latin, namely the pluperfect indicative (amaveram Latin amara Spanish) and pluperfect subjunctive (amavissem Latinamase Spanish). This present study of peninsular Spanish is a diachronic examination of the hypothesis that the –se form has gradually decreased in use while its opposing form has gradually increased in use resulting in a linguistic shift of the imperfect subjunctive. In this preliminary study of the –se/-ra forms from early Castilian to modern Spanish, special consideration will be given to contextual variables affecting the selection of –ra or –se forms. These variables include the type of literature and the differing levels of formality thereof (narrative, historical and scientific prose) in which the verb forms are found in order to analyze the selection of one form or another as a potential function of the class of literature. Another factor under examination is the placement of the verb within the protasis or apodosis of the sentence (the if or then clause) to analyze the degree to which the reality vs. the probable/hypothetical nature of an action affects the verb choice. A review of the related literature, the analysis of data samples, historic linguistic trends and correlations observed in the usage of the imperfect subjunctive will be presented.