आङोऽनुनासिकश्छन्दसि

Operational rule vidhi

In the context of संहिता and in the derivation of the वैदिक forms, when आङ् is followed by an अच् letter, आङ् becomes अनुनासिक and does not undergo sandhi with the अच् letter.

Word by word (padaccheda)

Target आङः ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacanaRight context छन्दसि 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, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

आङः छन्दसि

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)

आङः → / छन्दसि

Replace āṅaḥ with anunāsikaḥ when chandasi follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the adverb आ , is substituted in the छन्दस् the nasalised आँ , when a vowel follows it, and it retains its original form.
Thus अभ्र आं अपः (Rig V. 48.1); गभीर आं उग्रपुत्रे जिघांसतः (Rig VIII.67.11). Some read the word बहुलं into this sutra. Hence there is coalescence here: इन्द्रो बाहुभ्यामातरत् = आ अतरत् ॥

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