तदोः सः सावनन्त्ययोः

Operational rule vidhi

For the words of त्यदादिगण, a तकार or a दकार which is not at end of the word is converted to सकार when followed by the सुँ-प्रत्यय.

Word by word (padaccheda)

Target तदोः ṣaṣṭhī · dvivacanaSubstitute / term सः prathamā · ekavacanaRight context सौ saptamī · ekavacanaTarget अनन्त्ययोः ṣaṣṭhī · dvivacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

तदोः सः सौ अनन्त्ययोः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

तदोः अनन्त्ययोः → सः / सौ

Replace tadoḥ anantyayoḥ with saḥ when sau follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the non-final त् and द् of त्यद् etc. there is substituted स् in the Nominative 1st-Case singular.
As त्यद् + सु = त्य + अ + सु 7.2.102 = स्य + अ + सु 7.2.106 = स्यः 6.1.97. Similarly सः from तद्, एषः from एसद् as एतद् + सु = एत + अ + स् 7.2.102 = एस + अ + स् 7.2.106 = एषः 6.1.97. So असौ from अदस् by the following sutra. Why do we say 'non-final'? Observe हे से, सा ॥ Had not this word been used, the case-affix would not be elided in the vocative, as then there would have been no short vowel, as required by 6.1.69.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.