Much uncertainty still exists about the possible role of a myriad of external and internal mediating variables such as language backgrounds and ethnicities in the process of corrective feedback (CF). To address the issue, 79 monolingual Persians and 79 bilingual Turkmens aged between 13 and 18 from two language institutes in Golestan Province, Iran participated in the study comprising three experimental and a control group each. Pre-tests revealed the learners’ grammatical errors based on which the experimental groups were provided with different strategies of metalinguistic CF, namely mid-focused oral metalinguistic CF (OMCF), written metalinguistic CF (WMCF), and oral/written metalinguistic CF (OWMCF) on their five most recurrent grammatical errors while the control groups received none. Following significant Kruskal-Wallis test results, Conover’s pairwise comparison test was employed to exhibit differences among CF strategies. In monolingual Persians and bilingual Turkmens, all the experimental groups significantly outperformed the control groups. Mid-focused OMCF and OWMCF had significant effects on the reduction of Persians’ errors, but for Turkmens, all the CF strategies exerted a significant reduction of errors. The effectiveness rank of the CF strategies investigated through Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) selected OMCF as the best strategy for monolingual Persians while for bilingual Turkmens, WMCF ranked first. This ranking was highly stable for 10000 iterations changing the weights up to 50%. The outcomes of the research might be of help to EFL learners, EFL teachers of bilingual learners, syllabus designers, and materials developers while opening doors to further pertinent studies.

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