Grammatical reasoning is a hallmark of grammatical thought, as linguistic needs led to the establishment of grammatical rules to protect the language from error. Therefore, we cannot neglect or overlook grammatical reasoning as it is considered one of the essential pillars of grammar studies, enabling us to understand and study grammatical rules effectively. The reasons behind inflection received ample attention in the book Al-Mukhtaraa'; thus, to explore Al-A'lam's approach to grammatical reasoning and his efforts in this regard, the study delves into the grammatical justification for inflected nouns according to Al-A'lam Al-Shantamari (D. 476 AH) in his book Al-Mukhtaraa' fi Idha'at Sara'ir Al-Nahw. The research concludes that Al-A'lam presented innovative justifications in Al-Mukhtaraa', while also employing arguments previously proposed by other grammarians, as well as justifications that closely resemble those of his predecessors. We divide the research into an introduction, an entry, a main chapter, and a conclusion. The introduction provides an overview of the grammatical reasoning for inflected nouns according to Al-A'lam Al-Shantamari in Al-Mukhtaraa'. The entry defines the concepts of reasoning and justification, along with the categories of justifications according to grammarians. The study's main chapter examines a selection of inflected nouns that Al-A'lam justified in Al-Mukhtaraa'. The conclusion summarizes the main research findings.