न मपूर्वोऽपत्येऽवर्मणः

Operational rule vidhi

In a अङ्ग (stem) in अन् with a preceding म् with the exception of वर्मन् , the ending अन् does not remain unchanged before the Patronymic affix अण् .

Word by word (padaccheda)

Indeclinable न avyayaSubstitute / term मपूर्वः prathamā · ekavacanaRight context अपत्ये saptamī · ekavacanaTarget अवर्मणः ṣaṣṭhī · ekavacana

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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").
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

न मपूर्वः अपत्ये अवर्मणः

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

Commentary

In a अङ्ग (stem) in अन् with a preceding म् with the exception of वर्मन् , the ending अन् does not remain unchanged before the Patronymic affix अण् ।
Thus सुषाम्णोऽपत्यं = सौषामः, चान्द्रसामः from चन्द्रसामन् ॥ Why do we say 'preceded by म्'? Observe सोत्वनः, preceded by व ॥ Why do we use 'Patronymic'? Observe चर्मणा परिवृतो रथः = चामर्णः, the अन् not being changed by 6.4.167. Why do we say 'with the exception of वर्मन'? Observe चक्रवर्मणोऽपत्यं = चाकवर्मणः ॥

Vart:- Optionally so in हितनामन्, as हितनाम्नोऽपत्यं = हैतनामः or हैतनामनः ॥

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