The Ashtadhyayi is one of the earliest known grammars of Sanskrit, although Pāṇini refers to previous texts like the Unadisutra, Dhatupatha, and Ganapatha. It is the earliest known work on linguistic description, and together with the work of his immediate predecessors (the Niruktas, Nighantus, and Pratishakyas) stands at the beginning of the history of linguistics itself. His theory of morphological analysis was more advanced than any equivalent Western theory before the mid 20th century, and his analysis of noun compounds still forms the basis of modern linguistic theories of compounding, which have borrowed Sanskrit terms such as bahuvrihi and dvandva.
Pāṇini's comprehensive and scientific theory of grammar is conventionally taken to mark the end of the period of Vedic Sanskrit, introducing the period of Classical Sanskrit.
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Special thanks to sanskritdocuments.org for providing the resources.