The success of various grammar-teaching approaches remains one of the key concerns in second language education. This study investigated the relative effects of Guided Discovery Learning (GDL) and Test–Teach–Test (TTT) in the development of grammatical knowledge among intermediate Iranian EFL learners, particularly in the acquisition of tense and article systems. A quasi-experimental pretest–posttest design was adopted, in which two intact classes (N = 34) were randomly assigned to GDL and TTT conditions. The two groups received sixteen instructional sessions delivered by the same teacher using parallel teaching materials. Grammatical knowledge was assessed using researcher-developed pretests and posttests created according to the institute’s curriculum requirements. ANCOVA analysis, controlling for pretest scores, revealed that while both groups made significant gains, the GDL group had higher posttest scores (F(1, 31) = 12.40, p = .001, ηp² = .34). These results indicate that inductive, learner-focused approaches to instruction may result in greater gains in grammar mastery than structured diagnostic teaching cycles. The results of this study demonstrate the possible benefit of guided discovery in EFL contexts. Future studies could build on the findings of this study by using larger samples and investigating the effectiveness of these approaches in technology-mediated learning environments.