Before an affix beginning with a vowel there are substituted for u of nu , the characteristic of the roots of the svādigaṇaḥ (5th Conjugation) for the final i , ī , u and ū of a root, as well as for ū of bhrū , the iy (for i or ī) and uv (for u or ū).,
Thus आप्नुवन्ति, राध्नुवन्ति, शक्नुवन्ति, from roots of the 5th class. चिक्षियतुः ललुवतुः, लुलुवुः, नियौ, नियः, लुवौ, लुवः and भ्रुवौ and भ्रुवः ॥
Why do we say 'before an affix beginning with a vowel'? Observe आप्नुयात्, शक्नुयात्, राध्नुयात् ॥ Why \of श्नु &c.\? Observe लक्ष्यै, लक्ष्याः, वध्वै, वध्वाः ॥ Why \of इ and उ\? Observe चक्रतुः, चक्रुः where the vowel is ऋ ॥
Vart:- The Guna 7.3.84 and Vriddhi 7.2.115 however take place to the supersession of इयङ् and उवङ् as from चि - चयनम् and चायकः, लू - लवनम् and लावकः, with ल्युट् and ण्वुल् ॥
Vart:- In the Chhandas there is deversely the substitution of इयङ् and उवङ् in the case of तन् &c. Thus तन्वं पुषेम or तनुवं पुषेम ॥ विषुवं पुषेम, स्वर्गो लोकः, सुवर्गो लोकः, त्र्यम्बकं यजामहे, त्रियम्बकं यजामहे ॥,