विभाषा सपूर्वस्य

Operational rule vidhi

In forming the feminine with the affix ङीप् , न् is optionally the substitute of the final इ of पति when the latter is preceded by another vowel.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaTarget स- पूर्वस्य ṣaṣṭhī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

स-पूर्वस्य

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)

स- पूर्वस्य → optional

Replace sa-pūrvasya with vibhāṣā, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

In forming the feminine with the affix ङीप् , न् is optionally the substitute of the final इ of पति when the latter is preceded by another vowel.
The word सपूर्वस्य means when the word पति is not an upasarjana, but stands at the end of a compound. Thus वृद्धपतिः or वृद्धपत्नी; स्थूलपतिः or स्थूलपत्नी ॥ This is an example of aprapta-vibhasha. Because it is free from the limitations of 'yajna-sanyoga' of the last sutra. Why do we say when it is preceded by another word? Observe पतिरियं ब्राह्मणी ग्रामस्य, this Brahman lady is the mistress of the village.' Here the feminine is पति and not पत्नी ॥

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