The oldest attested full case and tense system grammar of a language from the Indo-European language family. Comparing with Classical Sanskrit, Vedic Sanskrit had a subjunctive absent in Panini's grammar and generally believed to have disappeared by then at least in common sentence constructions. All tenses could be conjugated in the subjunctive and optative moods, in contrast to Classical Sanskrit, with no subjunctive and only a present optative. However, the old first-person subjunctive forms were used to complete the Classical Sanskrit imperative. The three synthetic past tenses (imperfect, perfect and aorist) were still clearly distinguished semantically in (at least the earliest) Vedic. A fifth mood, the injunctive, also existed.

Long-i stems differentiate the Devi inflection and the Vrkis inflection, a difference lost in Classical Sanskrit.

• The subjunctive mood of Vedic Sanskrit was also lost in Classical Sanskrit. Also, there was no fixed rule about the use of various tenses (luṇ, laṇ and liṭ).
• There were more than 12 ways of forming infinitives in Vedic Sanskrit, of which Classical Sanskrit retained only one form.
• Nominal declinations and verbal conjugation also changed pronunciation, although the spelling was mostly retained in Classical Sanskrit. E.g., along with the Classical Sanskrit's declension of deva- as devas—devau—devās, Vedic Sanskrit additionally allowed the forms daivas—daivā—daivāsas. Similarly Vedic Sanskrit has declined forms such as asmai, tvai, yuṣmai, tvā, etc. for the 1st and 2nd person pronouns, not found in Classical Sanskrit. The obvious reason is the attempt of Classical Sanskrit to regularize and standardize its grammar, which simultaneously led to a purge of Old Proto-Indo-European forms.
• To emphasize that Proto-Indo-European and its immediate daughters were essentially end-inflected languages, both Proto-Indo-European and Vedic Sanskrit had independent prefix-morphemes. Such prefixes (especially for verbs) could come anywhere in the sentence, but in Classical Sanskrit, it became mandatory to attach them immediately before the verb.