processability theory: stage-like development of “3 sg-s” and “cancel auxsecond” in iranian efl learners’ writing performance

processability theory (pt) is a second language acquisition (sla) theory developed to explain developmental sequences in sla as well as some other phenomena (pienemann, 1998a).processability has been a main concern in sla research since 1990s. following the agenda of processability theory and through analyzing the written performance of iranian efl learners’ writing performances, the present research studied the acquisition of “3 sg-s” and “cancel auxsecond or cancel inversion” across five proficiency levels, from elementary to advanced, and compared it with the stage-like development model of morpho-syntactic structures proposed by pienemann (1998a). the study followed a descriptive method of research, and the data was collected from 350 participants in five different proficiency levels from elementary, pre-intermediate, intermediate, upper-intermediate and advanced. the participants were asked to provide samples of their written performance on different tasks including an introduction task, a habitual action task, a story retelling task, a picture description task, and a composition and communication task. the data in this research was analyzed both qualitatively, in order to recognize and classify the type and order of the morpho-syntactic structures, and quantitatively, by calculating means. the results of kruskal-wallis test revealed that both “3 sg-s” and “cancel auxsecond,” as two morpho-syntactic features, emerge very early in the language learners’ performance. very similarly, the competence of the learner grows stronger in concern with these variables through the higher proficiency levels. these findings imply that pt is valid to a considerable extent for iranian efl learners, as well.