Five chronologically distinct strata can be identified within the Vedic language (Witzel 1989).
Ṛgvedic. The Ṛgveda retains many common Indo-Iranian elements, both in language and in content, that are not present in any other Vedic texts. Its creation must have taken place over several centuries, and apart from the latest books (1 and 10), it must have been essentially complete by around 1200 BCE.
Mantra language. This period includes both the mantra and prose language of the Atharvaveda (Paippalada and Shaunakiya), the Rigveda Khilani, the Samaveda Samhita (containing some 75 mantras not in the Rigveda), and the mantras of the Yajurveda. These texts are largely derived from the Rigveda, but have undergone certain changes, both by linguistic change and by reinterpretation. Conspicuous changes include change of viśva "all" to sarva, and the spread of kuru- (for Rigvedic kṛno-) as the present tense form of the verb kar- "make, do". This period corresponds to the early Iron Age in north-western India (iron is first mentioned in the Atharvaveda), and to the kingdom of the Kurus, dating from about the twelfth century BCE.
Samhita prose (roughly 1100 BCE to 800 BCE). This period marks the beginning collection and codification of a Vedic canon. An important linguistic change is the complete loss of the injunctive and of the grammatical moods of the aorist. The commentary part of the Black Yajurveda (MS, KS) belongs to this period.
Around 500 BCE, cultural, political and linguistic factors all contribute to the end of the Vedic period. The codification of Vedic ritual reached its peak, and counter movements such as the Vedanta and early Buddhism emerged, using the vernacular Pali, a Prakrit dialect, rather than Sanskrit for their texts. Darius I of Persia invaded the Indus valley and the political center of the Indo-Aryan kingdoms shifted eastward, to the Gangetic plain. Around this time (4th century BCE), Panini fixes the grammar of Classical Sanskrit.
This section treats the distinguishing features of Vedic Sanskrit — see Classical Sanskrit for a general account.
Sound changes between Proto-Indo-Iranian and Vedic Sanskrit include loss of the voiced sibilant z.
Vedic Sanskrit had a bilabial fricative[ɸ], called upadhmānīya, and a velar fricative [x], called jihvamuliya. These are both allophones of visarga: upadhmaniya occurs before p and ph, jihvamuliya before k and kh. Vedic also had a retroflex l for retroflex l, an intervocalic allophone of ḍ, represented in Devanagari with the separate symbol ळ and transliterated as ḷ or ḷh. In order to disambiguate vocalic l from retroflex l, ISO 15919 transliterates vocalic l with a ring below the letter, l̥. (Vocalic r is then also represented with a ring, r̥, for consistency and to disambiguate it additionally from the retroflex ṛ and ṛh of some modern Indian languages.)
Vedic Sanskrit had a pitch accent. Since a small number of words in the late pronunciation of Vedic carry the so-called "independent svarita" on a short vowel, one can argue that late Vedic was marginally a tonal language. Note however that in the metrically restored versions of the Rig Veda almost all of the syllables carrying an independent svarita must revert to a sequence of two syllables, the first of which carries an udātta and the second a (so called) dependent svarita. Early Vedic was thus definitely not a tonal language but a pitch accent language. See Vedic accent.
Pāṇini gives accent rules for the spoken language of his (post-Vedic) time, though there is no extant post-Vedic text with accents.
The pluti vowels (trimoraic vowels) were on the verge of becoming phonological during middle Vedic, but disappeared again.
Vedic Sanskrit differs from Classical Sanskrit to an extent comparable to the difference between Homeric Greek and Classical Greek. Tiwari ( 2005) lists the following principal differences between the two:
Vedic Sanskrit had a voiceless bilabial fricative (/ɸ/, called upadhmānīya) and a voiceless velar fricative (/x/, called jihvāmūlīya)—which used to occur when the breath visarga (अः) appeared before voiceless labial and velar consonants respectively. Both of them were lost in Classical Sanskrit to give way to the simple visarga.
Vedic Sanskrit had a retroflex lateral approximant (/ɭ/) (ळ) as well as its aspirated counterpart /ɭʰ/ (ळ्ह), which were lost in Classical Sanskrit, to be replaced with the corresponding plosives /ɖ/ (ड) and /ɖʱ/ (ढ). (Varies by region; Vedic pronunciations are still in common use in some regions, e.g. southern India, including Maharashtra.)
The pronunciations of syllabic /ɻ̩/ (ऋ), /l̩/ (लृ) and their long counterparts no longer retained their pure pronunciations, but had started to be pronounced as short and long /ɻi/ (रि) and /li/ (ल्रि).
The vowels e (ए) and o (ओ) were actually realized in Vedic Sanskrit as diphthongs/ai/ and /au/, but they became pure monophthongs/eː/ and /oː/ in Classical Sanskrit.
The vowels ai (ऐ) and au (औ) were actually realized in Vedic Sanskrit as hiatus/aːi/ (आइ) and /aːu/ (आउ), but they became diphthongs/ai/ (अइ) and /au/ (अउ) in Classical Sanskrit.
The Prātishākhyas claim that the dental consonants were articulated from the root of the teeth (dantamūlīya), but they became pure dentals later. This included the /r/, which later became retroflex.
Vedic Sanskrit had a pitch accent which could even change the meaning of the words, and was still in use in Panini's time, as we can infer by his use of devices to indicate its position. At some latter time, this was replaced by a stress accent limited to the second to fourth syllables from the end.
Vedic Sanskrit often allowed two like vowels to come together without merger during Sandhi.
Vedic 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.
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 twelve 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 devaḥ—devau—devāḥ, Vedic Sanskrit additionally allowed the forms devaḥ—devā—devāsaḥ. Similarly Vedic Sanskrit has declined forms such as asme, tve, yuṣme, 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.
Proto-Indo-European and its immediate daughters were essentially end-inflected languages in which what would later become bound prefixes were still independent morphemes. Such morphemes (especially for verbs) could come anywhere in the sentence, but in Classical Sanskrit, it became mandatory to attach them immediately before the verb; they, then, ceased being independent morphemes and became prefix-morphemes bound to the beginnings of verbs. There was a similar development from Homeric Greek to Classical Greek: see tmesis.