पुरोऽग्रतोऽग्रेषु सर्तेः

Operational rule vidhi

The affix ट comes after the verb सृ 'to move' when the words in composition with it are पुरः , अग्रतः and अग्रे , all meaning 'in front or before'.

Word by word (padaccheda)

Right context पुरस्- अग्रतः- अग्रेषु saptamī · bahuvacanaLeft context सर्त्तेः pañcamī · ekavacana

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").
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

पुरस्-अग्रतः-अग्रेषु सर्त्तेः

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

Commentary

The affix ट comes after the verb सृ 'to move' when the words in composition with it are पुरः , अग्रतः and अग्रे , all meaning 'in front or before'.
As पुरः सरति = पुरःसरः 'who goes in front, a harbinger'. So also अग्रतः सरः and अग्रसरः 'going in front, a leader'.

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