1. Vowel Sandhi:

   • When two vowels come together, the first vowel undergoes certain changes.
   • A + a = ā
   • A + i = āi
   • A + u = āu
   • E + a = o
   • O + a = o

2. Consonant Sandhi:

   • When certain consonants combine, they undergo changes in pronunciation.
   • Voiced aspirated stops (bh, dh, gh) become voiceless aspirated stops (ph, th, kh) before voiceless consonants.
   • Voiced stops (b, d, g) become their corresponding nasals (m, n, ṅ) before nasal consonants.
   • Some consonants assimilate to the following consonant, changing their pronunciation accordingly.

3. Visarga Sandhi:

   • Visarga (ḥ) combines with a following consonant or vowel and undergoes changes.
   • After certain vowels, visarga changes to the corresponding aspirated consonant. For example, aḥ + ka = akkha.
   • Before certain consonants, visarga may become anusvara (ṁ) or just assimilate to the following consonant.

4. Anusvara Sandhi:

   • Anusvara (ṁ) undergoes changes when followed by certain consonants or vowels.
   • Before stops (except voiced aspirates), anusvara becomes the corresponding nasal.
   • Before certain vowels, anusvara may assimilate and change its pronunciation.

5. Consonant Cluster Sandhi:

   • When two consonants come together, they might undergo simplification or assimilation.
   • Some clusters simplify by dropping one of the consonants.
   • Nasals and semivowels often assimilate to the following consonant.

Retroflexion is a significant aspect of Sanskrit pronunciation, especially in relation to internal sandhi. Retroflex sounds involve the curling back of the tongue, producing a distinct set of consonants. Here are some rules related to retroflexion in Sanskrit internal sandhi:

1. Retroflexion in Consonant Clusters:

   • When a dental (d, t) or a palatal (c, ch, j, jh, ñ) consonant comes before a retroflex consonant, the retroflexion is often assimilated by the preceding consonant.
     • Example: danta (tooth) + sthāna (place) becomes dantasthāna.

2. Retroflexion with Vowels:

   • When a vowel follows a retroflex consonant, the retroflexion may influence the pronunciation of the vowel.
   • Retroflexion of the preceding consonant may cause the vowel to become a retroflex vowel.
     • Example: tar (to cross) + ag (fire) becomes tarag with a retroflex vowel.

3. Retroflex Nasals and Laterals:

   • Retroflex nasal (ṇ) and retroflex lateral (ḷ) have specific rules in sandhi.
   • Before certain sounds, retroflex nasal may assimilate or change.
   • Retroflex lateral may be influenced by the following sounds.

4. Retroflex Consonants in Compounds:

   • In compound words, the retroflexion of the second element may be assimilated to the first element.
     • Example: gaja (elephant) + āsana (seat) becomes gajāsana.