Abstract The delicate issue of the development of the monolingual, bilingual, and trilingual speakers learning English as a foreign language was considered in this study. The idea of learning a foreign language and the preparation for participating in the TOEFL test was combined to determine the degree of achievement and progress between monolingual, bilingual and trilingual speakers. Consequently, proposals to integrate the study of foreign language learning and trilingualism have been made as a means of describing some of the ways in which monolingual, bilingual, and trilingual speakers differ in their progress of mastering a foreign language which lead to their success in the TOEFL exam. This study was mainly carried out to test this presupposition that those who have acquired more than one native language might be more successful in learning another one after puberty. The subjects of the study consist of sixty-four students: thirty monolingual, sixteen bilingual, and sixteen trilingual students studying English as a foreign language with the aim of passing the TOEFL test. Their native languages were Persian, Persian and Kurdish or Turkish, Persian and Kurdish and Turkish. The subjects were randomly placed into two groups. The lecturers were the same. The covered topics were grammar, listening and reading comprehension. A pre-test was carried out before the intensive course and a TOEFL test was the final step of the instruction period. The teaching period was four months full-time instruction. The findings did not show the superiority of bilingual and trilingual speakers over the monolingual ones. In fact, the monolingual subjects did slightly better than the other groups. However, the t-test and other statistical analysis proved that the difference is not significant. Consequently, it is possible to infer that after the age of puberty having more than one native like language cannot be of great help in learning a foreign language. The implications of the results might be in methodology addressed in terms of appropriate training content, and the needs for an educationally and linguistically methodology as effective means in teaching English as a foreign language.

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