The ancient Indian grammarian who wrote the Ashtadhyayi was Panini.
Panini was a Sanskrit grammarian whose work is usually placed around the middle of the 1st millennium BCE, though his exact dates remain uncertain. His Ashtadhyayi, meaning “Eight Chapters,” presents nearly 4,000 concise rules for analyzing and generating correct Sanskrit forms. Its systematic treatment of sounds, word formation, compounds, and syntax made it one of the most influential works in the history of linguistics.
The text is more than a list of definitions. Its rules operate in carefully ordered sequences, with technical markers and conventions that allow complex words and sentences to be derived from basic elements. Panini described the Sanskrit of learned speakers while also preserving information about older Vedic usage.
Panini’s work influenced later Indian grammatical traditions, including commentaries by Katyayana and Patanjali. That relationship causes a frequent mix-up: Katyayana wrote important supplementary observations, and Patanjali composed the Mahabhashya, a major commentary, but neither wrote the Ashtadhyayi itself. Yaska, another earlier grammarian and etymologist, is associated with the Nirukta.