अपगुरो णमुलि

Operational rule vidhi

There is optionally the substitution of आ in the room of the diphthong of the root गुर् 'to exert' when preceded by the preposition अप and taking the affix णमुल्.

Word by word (padaccheda)

Target अपगुरः ṣaṣṭhī · ekavacanaRight context णमुलि saptamī · 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").

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

अपगुरः णमुलि

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

Commentary

There is optionally the substitution of आ in the room of the diphthong of the root गुर् 'to exert' when preceded by the preposition अप and taking the affix णमुल्।
The affix णमुल् forming the Absolutive Participle is added by 3.4.22, and the word formed by it is repeated. Thus अपगारमपगारम् or अपगोरमपगोरम् ॥ So also this affix is added by 3.4.53: as, अस्यपगारं युध्यन्ते or अस्यपगोरं युध्यन्ते 'they are fighting with raised swords'.

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