Within the context of task-based instruction (TBI), it is now widely recognized that focus on form is required to boost second language learners’ production accuracy of second language (L2) forms and to peak learners’ attainment in L2. This dissertation consists of 4 interconnected but independent experiments investigating the effects of: Full Form-Focused Instruction, Focused and Unfocused tasks, First Language Metalinguistic Comments, and Place of Focus on Form in a task sequence on Iranian EFL learners’ recognition of English grammatical forms in terms of accuracy and durability. To this end, 3 quasi-experimental investigations were designed and seven university intact classes were randomly assigned to experimental and comparison groups. All groups were homogenized in terms of their language proficiency and their grammar knowledge before the onset of the instruction. The participants’ scores performing the given tasks were compared via ANCOVA, t test, and Pairwise Post Hoc Comparisons test. The results of Experiment 1 indicated that L1 metalinguistic comments lead to more durable effects than L2 metalinguistic comments and that the use of L1 helped learners, especially those whose language proficiency is below lower-intermediate, to become more cognitively conscious and linguistically aware of the grammar features. In experiment 2, the results indicated that both input flood tasks and Jigsaw tasks can immediately after the instruction promote learners’ recognition of English past tense marker /-ed/. However, the effects of input flood tasks gradually diminished after the instruction. The findings of the third experiment demonstrated that full form-focused instruction was more effective than form-focused instruction for promoting accurate and durable recognition of present perfect tense. As for the last experiment, the findings indicated that the presentation of focus on form activities in the posttask phase was more beneficial than the time these instructions app
