6.4.87 हुश्नुवोः सार्वधातुके

Word by word (padaccheda)

Target हु- श्नुवोः ṣaṣṭhī · dvivacanaRight context saptamī · 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").

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Full reading

हु-श्नुवोः

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

Commentary

The semi-vowel व् is substituted for the उ of हु and for that of नु (the characteristic of स्वादिगणः (5th Conjugation), before a सार्वधातुक affix -- तिङ्शित्सार्वधातुकम् 3.4.113, beginning with a vowel when the अङ्ग (stem) consists of more than one syllable and the उ is not preceded by a conjunct consonant.
Thus हु — जुह्वति, जुह्वतु; अजुह्वन्, so also with सु — as सुन्वन्ति, सुन्वन्तु, असुन्वन् ॥ Why do we say "of हु and श्नु formed stems"? Observe योयुवति, रोरुवति, from Intensive bases, by the elision of the यङ् affix, The यङ् is elided in the secular literature also, by the implication (jnapaka) of this aphorism, for no counter-example can be formed of a root consisting of more than one syllable and ending in उ, not preceded by a conjunct consonant and followed by a Sarvadhatuka affix, unless the intensive roots with the elision of यङ् be taken. Nor can we get examples from the Chhandas, for the preceding rule applies only to ardhadhatuka affixes. Why do we say before a Sarvadhatuka? Observe जुहुवतुः, जुहुवुः 6.4.77. So also not in आप्नुवन्ति and राध्नुवन्ति as the उ is preceded by a conjunct consonant.

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