8.2.42
रदाभ्यां निष्ठातो नः पूर्वस्य च दः
Operational rule vidhi
After र and द for the त of the Participial suffix त and तवत् there is substituted न and the same substitution takes place also for the preceding द.
Word by word (padaccheda)
Left context रदाभ्याम् pañcamī · dvivacanaTarget निष्ठातः ṣaṣṭhī · ekavacanaSubstitute / term नः prathamā · ekavacanaTarget पूर्वस्य ṣaṣṭhī · ekavacanaIndeclinable च avyayaTarget दः ṣaṣṭhī · ekavacana
- Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
- 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.
- Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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Full reading
रदाभ्याम् निष्ठातः नः पूर्वस्य च दः
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
निष्ठातः पूर्वस्य दः → नः / रदाभ्याम्
Replace niṣṭhātaḥ pūrvasya daḥ with naḥ when it comes after radābhyām. Inherited words (anuvṛtti) may add further conditions.
Commentary
After र and द for the त of the Participial suffix त and तवत् there is substituted न and the same substitution takes place also for the preceding द।
After र :- आस्तीर्णम्, विस्तीर्णम्, विशीर्णम्, निगीर्णम्, अवगूर्णम् ॥
After द :- भिन्नः, भिन्नवान्, छिन्नः, छिन्नवान् from भिद् and छिद् ॥
Why do we say "after र and द"? Observe कृतः, कृतवान् ॥ The word र here does not denote the common sound र, which would include ऋ also, but the consonant र ॥ But even if र् be taken a common sound-name including र् and ऋ, yet the न् change does not take place in कृत &c, because between त and the र्-sound, there intervenes vowel-sound इ, for ऋ is sounded not like pure र्, but र् + a vowel sound.
Why do we say "of the Participial suffix"? Observe कर्त्ता, हर्त्ता ॥
Why do we say "for the त"? Observe चरितम्, हरितम् ॥ Here the त of the Nishtha does not follow immediately after र्, the augment इट् intervenes.
Why do we say "of the preceding"? The succeeding द will not be changed. As भिन्नवद्भ्याम्, भिन्नवद्भिः ॥
In the word कार्तिः the descendant of कृतः the त of Nishtha is immediately preceded by र्, but no change has taken place, because the Vriddhi, by which कृ is changed to कार्, is Bahiranga and consequently asiddha, and for the purposes of न change, the र् so obtained is invalid.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.