This study aims at following morphological and grammatical reasoning `Illas` . Through presentation and analysis of grammarians` opinions and with quotations from classical Arabic, especially from the holy Quraan. It has become quite clear that the holy Quraan. It has become quite clear that the phenomenon of reasoning is necessary for realization of teaching goals of grammar. Teaching reasonings `Iuas` enables us to know the Arabic tongue. Some of these reasonings `Illas` are necessary for achieving some linguistic end. By the analogical reasonings `Iuas` we can imitate the Arabs and measure on their speech and thus ensure the language continuity, life and development. The study falls on three chapters, the first of which deals with nature and history of reasonings `Iuas` as well as the dictionary meaning of reasoning. By grammatical linguistic reasoning `Iuas` is meant the interpretation of linguistic phenomena and thereby penetrate to the rationale behind them, and hence explain the reasons that made them the way they are. In the second chapter, which is considered the actual study of the phenomena of reasonings, the morphological changes that words undergo have been investigated. The third chapter handles grammatical reasonings `Iuas`, expatiating on inflected forms, invariable forms the nominative case, the accusative case, the genetive case and quiescence particles. The research concludes with most important findings at through the study