श्वयुवमघोनामतद्धिते

Operational rule vidhi

The भसंज्ञक अङ्ग of श्वन् , युवन् and मघवन् undergoes सम्प्रसारणम् when followed by a प्रत्यय which is not a तद्धितप्रत्यय.

Word by word (padaccheda)

Target श्व- - मघोनाम् ṣaṣṭhī · bahuvacanaRight context अतद्धिते 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

श्व- -मघोनाम् अतद्धिते

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

Commentary

The व of 1. श्वन् 2. युवन् and 3. मधवन् becomes vocalised, but not before a तद्धित 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.

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