This study deals with the duality of definiteness and indefiniteness in language and its impact on the Arabic sentence structure on the assumption of the adaptation of the abstract mental thought and actual utterance according to the rules of the Arabic syntax since these rules usually differentiate between the definite and the indefinite by grammatical position and function.
Arab grammarians say that the indefinite is stronger than the definite because things are indefinite and then, become definite. This paper tackles definiteness and indefiniteness in English as well their effects on English sentence structure by grammatical position, expansion and function .
Finally, it is shown that the duality of definiteness and indefiniteness plays an important role on the Arabic and English sentence structure by grammatical position, sentence expansion and grammatical function.