This study aims to investigate the effectiveness of a Cognitive Linguistics (CL) usage-based approach in teaching countability to Iranian EFL learners, compared to a rule-based approach. Based on recent studies in CL, countability can be seen as a general feature of English nouns rather than an internal fixed characteristic. This study aims to examine whether teaching countability using this usage-based approach can improve learners' grammatical competence of countability in EFL contexts. The study involved 33 EFL Iranian learners in a high school where they had registered in one of the randomly assigned instruction groups. Within the same syllabus, the rule-based group were taught using rule-based material suggested by the official textbook, while the usage-based group were instructed by Cognitive Grammar enriched material. The participants were assessed on a pre-test and a post-test one week after the instruction, and the results were analyzed using a Man-Whitney-U analysis. Results showed that although both groups covered the same grammatical structures, only the CG group achieved statistically significant improvement. The between-group difference was significant (r = 0.45), and the CG group’s within-group gain corresponded to a large effect size (d ≈ 1.24). When compared with previous CL-based studies in other L1 contexts such as Japanese and Indonesian (d = 0.71 and d ≈ 0.62), the Iranian learners demonstrated notably larger gains. Given the small sample size and quasi-experimental design, these findings should be interpreted cautiously. Further research is recommended to confirm the robustness and broader applicability of CG-based instruction across diverse EFL settings.
