The va of 1. śvan 2. yuvan and 3. madhavan becomes vocalised, but not before a taddhita affix.,
Thus शुनः, शुना, शुने, यूनः, यूना, यूने, मघोनः, मघोना, मघोने ॥ But शौवनं मांसम्, यौवनं वर्त्तते, माघवनः स्थालीपाकः with Taddhita affixes. शौवन is formed by अञ् affix 4.3.154 and औ being added by 7.3.4 as श्वन belongs dvaradi class.
This vocalisation takes place of the nouns ending in न्, but not when they become feminine or do not end in न्, as युवतीः पश्य, मघवतः, मघवते, मघवता ॥ The word अनः of the next sutra, in a way, qualifies this sutra also.,