सर्वस्य सोऽन्यतरस्यां दि

Operational rule vidhi

The स is substituted for सर्व before a प्राग्दिशीय विभक्ति beginning with द.

Word by word (padaccheda)

Target सर्वस्य ṣaṣṭhī · ekavacanaSubstitute / term सः prathamā · ekavacanaRight context अन्यतरस्याम् saptamī · ekavacanaRight context दि saptamī · ekavacana

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

सर्वस्य सः अन्यतरस्याम् दि

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)

सर्वस्य → सः / अन्यतरस्याम् दि

Replace sarvasya with saḥ when anyatarasyām di follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

The स is substituted for सर्व before a प्राग्दिशीय विभक्ति beginning with द।
Thus सर्वदा or सदा ॥ When the affix is not Pragdisiya, there is no substitution : as, सर्वे ददाति = सर्वदा ब्राह्मणी 'a Brahmani who gives all'.

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