The study compared the pedagogical effects of early versus delayed Form Focused Instruction (EFFI vs. DFFI), both subsumed under Isolated Form Focused Instruction (IFFI), on the achievement of three target structures with relative degrees of complexity by monolinguals and bilinguals. Six intact Gilaki-Persian learners of English as L3 and six groups of Persian learners of English as L2 participated in the study. They were all male beginning learners of English in Iranian public high schools who followed a pretest-treatment-posttest procedure. Four groups (grade 7) received instruction for the simple structure; four other groups (grade 8) were taught the moderately complex structure and four groups (grade 9) were exposed to the highly complex structure instruction. Within each grade, one group of Gilaki and one group of Persian natives received EFFI while their native counterparts benefited DFFI. The overall results revealed that when the method of instruction was the same, Gilaki natives outperformed Persian natives both in the post and delayed tests regardless of complexity. The groups that received the simple structure via EFFI did better than their native counterparts instructed via DFFI in both the post and delayed tests though a significant difference was only observed in the latter test. In contrast, DFFI groups outperformed their native counterparts taught via EFFI on the fairly and highly complex structures in the post and delayed posttests. Further analysis of the data demonstrated that DFFI contributes better to the durability of gain effects for more complex structures regardless of linguistic background of the learners.
