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“Borrowing” is a lingual process that is studied in diachronic linguistics. In this process a language borrows elements from another language. This process usually occurs in areas that two languages make contact with each other. In a dialect spoken in South Khorasan the language borrowing happens. Arabs living in this part of Iran probably have immigrated in the early centuries of Islam. In this paper, the process of borrowing the verb “ast” and its varieties in “Sarab” village will be studied. In this village, this Farsi copular verb is used in three ways: est (with the phonetic, morphological and semantic transformation), hast (without any transformation) and mest (as a participle). In Arabic dialect of Sarab “ast” and its varieties are used in three modes of predictive, subjunctive and imperative. The use of Arabic identifiers in construction of “ast” is different in this dialect and its Farsi structure.