The distintion between the preterite and the imperfect is one of the most difficult grammatical items for spanish learning students. To determine a proper form between them, three properties compete: lexical aspect, grammatical aspect and discourse (foreground vs. background). This study intended to shed light on the hierarchic structure among them based on cognitive grammar. First, by applying “Pendulum Model” of Lopez Garcia (2005) for Spanish indicative simple tenses, [-tense] of the imperfect is explained in relation with the conditional (-ria). Second, the aspectual contrast [±perfective] between the preterite and the imperfect is specified with subfeatures like [±continuous], [±progressive] and [±initial point] which provide us comprehensible explanation of each use of both past tenses. Third, the relation between the lexical aspect and the grammatical aspect is explained with speaker`s viewing frame. The maximal viewing frame forces the verb to get the preterite while the restricted viewing frame forces the verb to get the imperfect. Finally, we pointed out that the contrast between foreground (figure) vs. background (ground) in the discourse participates in determining a proper past tense form as the last level.