The semi-vowel v is substituted for the u of hu and for that of nu (the characteristic of svādigaṇaḥ (5th Conjugation), before a sārvadhātuka affix -- tiṅśitsārvadhātukam [[3.4.113]], beginning with a vowel when the aṅga (stem) consists of more than one syllable and the u 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.,