A grammar expert is commonly referred to as a linguist or a grammarian.
Explanation: The term ‘linguist’ encompasses a broad range of language study, including grammar, syntax, semantics, and phonetics. Linguists often engage in the scientific analysis of language and its structure. On the other hand, a ‘grammarian’ specifically focuses on grammar—the rules that govern the structure of sentences in a language. Both terms reflect expertise in understanding and applying the rules of language, but they can have slightly different connotations depending on the context of their work.