8.4.58 अनुस्वारस्य ययि परसवर्णः

Word by word (padaccheda)

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

Full reading

अनुस्वारस्य परसवर्णः

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

Commentary

In the room of अनुस्वारा, when यय् follows, a letter homogenous with the latter is substituted.
Thus शङ्किता, शङ्कितुम्, शङ्कितव्यम्, उञ्छिता, कुण्डिता, नन्दिता, कम्पिता &c. These are from roots शकि शङ्कायाम्, उञ्छि उञ्छे, कुदिदाहे, टुनदि समृद्धौ, कपि चलने ॥ Here नुम् is added because they are उदित्, and this न् becomes anusvara by 8.3.24, and this anusvara is changed to ङ् when followed by a guttural क् &c, to ञ् when followed by a palatal च &c, and so on to ण्, न् and म् ॥

Why do we say 'when यय् follows'? Observe आक्रंस्यते, आचिक्रंस्यते ॥

In कुर्वन्ति, वृषन्ति, the न is not changed into ण, though required by 8.4.2. Because the णत्व is asiddha, and therefore by the prior rule 8.3.24, the न is first changed into anusvara 8.2.1. That anusvara is again changed into न by the present rule, न being homogeneous with त ॥ This change again being असिद्ध as if it had never taken place 8.2.1, the ण is never substituted for न ॥

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