The pu of the vikaraṇa u and śnu when not preceded by a conjunct consonant, may be elided optionally before a personal affix beginning with m or v,
Thus सुन्वः or सुनुवः, सुन्मः, सुनुमः, तन्वः, तनुवः, तन्मः, तनुमः ॥ The उ must belong to the affix, and should not be part of the root. Therefore not in युवः, युमः ॥
Why do we say 'not preceded by a conjunct consonant'? Observe शक्नुवः, शक्नुमः only. Though the elision word लुक् was understood in this sutra, the mention of लोप indicates that the final is only to be elided, and not the whole affix नु ॥ It is a general maxim that the words लुक्, श्लु and लुप् cause the elision of the whole affix, while 'lopa' will cause elision of the final letter only of the affix. If the whole affix be elided, we could not get सुन्वः &c., Moreover, in कुर्व and कुर्म formed by guna of कृ, there would have been no guna, had the word लुक् been used, for 1.1.63, would have prevented guna; but by using the word लोप we have such guna also by 1.1.62.,