The acquisition or learning of the rich verbal morphology is not an easy task for speakers whose language does not have it. Moreover, most learners of Spanish as L2 suffer some difficulties in learning verb conjugation due to some unexpected changes of radical vowel and agreement morpheme. It is hard for learners to store and use them effectively and quickly. We want to emphasize that the radical vowel change in finite forms like dipthongization (o>ue, e>ie, u>ue: poder, querer, jugar) or raising (e>i: pedir) is related to the morphophonological rules and this kind of vowel alternation should be treated as a rule-governed phenomenon. It is well known that the verb estar, conventionally treated as a irregular verb, never shows a radical vowel change but shows the irregular pattern in the agreement morphemes. In this case, learners store its pattern of agreement morphemes as a irregular one because they are not predictable according to the morphophonological or morphosyntactic property of the verbs with ending ‘-ar’ in Spanish language. We try to show that learning should not be directed to the lexical base [Root + TV(Theme Vowel, Vocal Tematica)](cf. Bermudez-Otero(2013), but to the structure of [Root + agreement morpheme](cf. Embick(2010, 2012) under the assumption that all the finite and non-finte forms are derived from the structure [Root + abstract morpheme].