8.3.4 अनुनासिकात् परोऽनुस्वारः

Word by word (padaccheda)

Left context pañcamī · ekavacanaSubstitute / term परः prathamā · ekavacanaSubstitute / term अनुस्वारः prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

परः अनुस्वारः

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

Commentary

After what precedes रु , if we omit to substitute the nasal, then अनुस्वार shall be the augment.
The substitution of nasal is optional by 8.3.2. When nasal is not substituted, we add an anusvara to such vowel. The word अन्य should be read into the sutra to complete the sense, i.e. अनुनासिकात् अन्यो यो वर्णः "a letter other than a nasal", i. e. a letter for which nasal has not been ordained, and which stands before रु ॥

Thus 8.3.5, teaches substitution of the म् of सम् an anusvara would be added. As संस्कर्त्ता, संस्कर्त्तव्यम् ॥ Similarly 8.3.6, teaches रु substitution of the म् of पुम् ॥ An anusvara will be added here also, as, पुंस्कामा ॥ Similarly 8.3.7, teaches रु substitution of final म् ॥ Here also an anusvara will be added, as भवांश्चरति ॥

Some say "the word परः in the sutra means अन्यः, and so we need not supply the word अन्यः from outside". They say अनुनासिकात् परः = अनुनासिकादन्यः; i. e. the anusvara takes place in that alternative when there is no nasal. That anusvara is an augment and not a substitute. It is an augment to the vowel which precedes रु ॥

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