Processability Theory (PT) is a theory of second language acquisition (SLA) developed to explain developmental sequences in SLA as well as some other phenomena. Within the framework of Processability Theory (PT) and through analyzing the written performance of Iranian EFL learners, the present research focused on the acquisition of the morpho-syntactic structures of “do- s- v (o)” across five proficiency levels, from elementary to advance and compared it with the stage-like development model of morpho-syntactic structures proposed by Pienemann (2005a). The study followed a mixed method design and the data were collected from 350 participants in five different proficiency levels from elementary, pre-intermediate, intermediate, upper-intermediate and advanced; furthermore, 45 pre-intermediate to advanced students were interviewed. The participants were asked to provide samples of their written performance on different tasks such as introduction task, habitual action task, and story retelling task, picture description task, composition, and communication task; furthermore, they were interviewed on the same topics. The data in this research were analyzed both qualitatively to identify and classify the type and order of the morpho-syntactic structures in the written and oral data, and quantitatively through inferential statistics. The results of Kruskal-Wallis test revealed that “do-subject- verb was concordant with Pienemann’s (2005a) model. This finding implies that PT is valid for Iranian EFL learners, considerably. The findings of this research can be of benefit for language teachers, learners, and syllabus designers.
