शदेरगतौ तः

Operational rule vidhi

For the final of शद् is substituted त् in the Causative, when it does not mean 'to drive'.

Word by word (padaccheda)

Target शदेः ṣaṣṭhī · ekavacanaRight context अगतौ saptamī · ekavacanaSubstitute / term तः prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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

शदेः अगतौ तः

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)

शदेः → तः / अगतौ

Replace śadeḥ with taḥ when agatau follows. Inherited words (anuvṛtti) may add further conditions.

For the final of शद् is substituted त् in the Causative, when it does not mean 'to drive'.
As पुष्पाणि शातयति, फलानि शातयति, but गाः शादयति गोपालकः ॥

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