नगोऽप्राणिष्वन्यतरस्याम्

Operational rule vidhi

The Negative Particle is optionally retained unchanged in नगो when it does not mean a living animate being.

Word by word (padaccheda)

Substitute / term नगः prathamā · ekavacanaRight context अप्राणिषु saptamī · bahuvacanaRight context अन्यतरस्याम् saptamī · ekavacana

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").

Full reading

नगः अप्राणिषु अन्यतरस्याम्

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

Commentary

The Negative Particle is optionally retained unchanged in नगो when it does not mean a living animate being.
Thus नगाः or अगाः 'trees', or 'mountains', literally 'what do not move'. The affix ड is added to गम् ॥ Why do we say when not referring to animate beings. Observe अगः वृषलः शीतेन ॥ No alternative form is allowed here.

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