On the continuum of approaches to Teaching English as a Foreign Language, the Iranian educational system falls in the Grammar Translation extreme of the continuum rather than the Communicative extreme. Thinking in the first language forms the building blocks of language learning in Grammar Translation Method, mainly through bi-directional translation from and to the first language. Among the many questions in such educational systems is how the knowledge of the previous languages affects the subsequent languages. The question becomes more influential when the first two languages fall into different morphological and syntactic categories. This study is going to see how the Turkish- Persian bilinguals engaged in the Iranian educational system learn English. Since one difference between Persian and Turkish lies in the position of prepositions, the main focus of this study is on this aspect of learning. The hypothesis of the study is that the mother tongue (L1) is more influential in learning the subsequent languages.