Hirt wrote on German metres (Untersuchungen zur westgermanischen Verskunst, 1889), edited Schopenhauer's Parerga (1890), and then devoting himself to Indo-Germanic philology made special studies on accent, writing Der indogermanische Accent (1895) and Der indogermanische Ablaut, vornehmlich in seinem Verhältnis zur Betonung (1900). Hirt, who became professor at the University of Leipzig, made valuable contributions to Brugmann and Streitberg's Indogermanische Forschungen, on the morphology of case endings. In 1902, he published Handbuch der griechischen Laut- und Formenlehre, the first volume of a series of Indo-Germanic text-books of which he was editor. He is the author of the Indogermanische Grammatik, published in seven volumes between 1921 and 1937. Hirt made foundational contributions to the study of Proto-Indo-European languageaccent and ablaut.