stylistic:tone, structural:expansion
Phonological alternations, sandhi rules

Sanskrit deploys extensive phonological alternations on different linguistic levels through sandhi rules (literally, the rules of "putting together, union, connection, alliance"), similar to the English alteration of "going to" as gonna. The Sanskrit language accepts such alterations within it, but offers formal rules for the sandhi of any two words next to each other in the same sentence or linking two sentences. The external sandhi rules state that similar short vowels coalesce into a single long vowel, while dissimilar vowels form glides or undergo diphthongization. Among the consonants, most external sandhi rules recommend regressive assimilation for clarity when they are voiced. These rules ordinarily apply at compound seams and morpheme boundaries. In Vedic Sanskrit, the external sandhi rules are more variable than in Classical Sanskrit.

The internal sandhi rules are more intricate and account for the root and the canonical structure of the Sanskrit word. These rules anticipate what are now known as the Bartholomae's law and Grassmann's law. For example, states Jamison, the "voiceless, voiced, and voiced aspirated obstruents of a positional series regularly alternate with each other (p ≈ b ≈ bh; t ≈ d ≈ dh, etc.; note, however, c ≈ j ≈ h), such that, for example, a morpheme with an underlying voiced aspirate final may show alternants with all three stops under differing internal sandhi conditions". The velar series (k, g, gʰ) alternate with the palatal series (c, j, h), while the structural position of the palatal series is modified into a retroflex cluster when followed by dental. This rule creates two morphophonemically distinct series from a single palatal series.

Vocalic alternations in the Sanskrit morphological system is termed "strengthening", and called guṇa and vr̥ddhi in the preconsonantal versions. There is an equivalence to terms deployed in Indo-European descriptive grammars, wherein Sanskrit's unstrengthened state is same as the zero-grade, guṇa corresponds to normal-grade, while vr̥ddhi is same as the lengthened-state. The qualitative ablaut is not found in Sanskrit just like it is absent in Iranian, but Sanskrit retains quantitative ablaut through vowel strengthening. The transformations between unstrengthened to guṇa is prominent in the morphological system, states Jamison, while vr̥ddhi is a particularly significant rule when adjectives of origin and appurtenance are derived. The manner in which this is done slightly differs between the Vedic and the Classical Sanskrit.

Sanskrit grants a very flexible syllable structure, where they may begin or end with vowels, be single consonants or clusters. Similarly, the syllable may have an internal vowel of any weight. Vedic Sanskrit shows traces of following the Sievers–Edgerton law, but Classical Sanskrit does not. Vedic Sanskrit has a pitch accent system (inherited from Proto-Indo-European) which was acknowledged by Pāṇini, states Jamison; but in his Classical Sanskrit the accents disappear. Most Vedic Sanskrit words have one accent. However, this accent is not phonologically predictable, states Jamison. It can fall anywhere in the word and its position often conveys morphological and syntactic information. The presence of an accent system in Vedic Sanskrit is evidenced from the markings in the Vedic texts. This is important because of Sanskrit's connection to the PIE languages and comparative Indo-European linguistics.

Sanskrit, like most early Indo-European languages, lost the so-called "laryngeal consonants (cover-symbol *H) present in the Proto-Indo-European", states Jamison. This significantly affected the evolutionary path of the Sanskrit phonology and morphology, particularly in the variant forms of roots.