Adult second-language learners of Spanish struggle with the acquisition of preterite and imperfect selection due to the overtly morphological representation of grammatical aspect. Prior studies have documented the effect of a default encoding without influence of the lexical aspect in the emergence of aspectual morphology, and have proposed the Default Past Tense Hypothesis (DPTH).

This study investigates the emergence of aspectual morphology by testing the DPTH and the effect of adverbials at interpreting grammatical aspect in this process of acquisition. Twenty-eight English-speaking learners of Spanish (beginning, intermediate and advanced) and twenty native-Spanish speakers are tested with two written comprehension tasks that assess the interpretation of habitual/imperfect and episodic/preterite readings of eventive verbs. The truth-value judgment task incorporates forty short stories with two summary sentences, from which participants must choose one as true. The grammaticality judgment task presents sixty-four sentences with temporal adverbials of position and duration, thirty-two are grammatical and thirty-two are ungrammatical. Participants must accept or reject them using a 5-point likert scale.

The findings indicate that the DPTH is partially supported by the statistical data showing a default marker, imperfect for beginning learners, and preterite for intermediate learners. This provides support to the argument of unsteady aspectual checking of [-bounded] in the spec of AspP and not necessarily by only checking [+past] in the TP for intermediate learners. The influence of the lexical aspect value of the verb is partially evident with advanced learners. Temporal adverbials play an important role at interpreting grammatical aspect with intermediate and advanced learners. Results show that beginning learners are not influenced by the presence of adverbials due to their inexperience with the Spanish aspectual morphology.

The findings also allow the confirmation of prior results about factors that influence the interpretation of preterite and imperfect. First, the instruction of aspectual morphology co-indexed with specific temporal adverbials, and second, that learners rely on lexical cues at the sentential level, while native speakers rely on discursive ones.