नित्यं सपत्न्यादिषु

Operational rule vidhi

In forming the feminine with the affix ङीप् the word पति always takes the substitute न् in the words like सपत्नि etc.

Word by word (padaccheda)

Substitute / term नित्यम् prathamā · ekavacanaRight context सपत्नी- आदिषु saptamī · bahuvacana

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").

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Full reading

नित्यम् सपत्नी-आदिषु

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

Commentary

In forming the feminine with the affix ङीप् the word पति always takes the substitute न् in the words like सपत्नि etc.
This enjoins the necessary and not optional substitution of न् for the final इ of पति, before the affix ङीप्, in the cases of certain words. The word 'nitya' is used in the aphorism for the sake of precision. Thus सपत्नी, एकपत्नी ॥ The compound of the following words with पत्नी are governed by this rule:- 1 समान 'equal' 2 एक 'one', 3 वीर 'hero', 4 पिण्ड 'a cake', 5 भ्रातृ 'brother' 6 पुत्र 'a son', and 7 दास 'a slave' (this only in the Chhandas).

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