अतोऽम्

Operational rule vidhi

The सुँ and अम् attached to an अकारान्त नपुंसक अङ्ग are converted to अम्.

Word by word (padaccheda)

Left context अतः pañcamī · ekavacanaSubstitute / term अम् prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

अतः अम्

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

Commentary

After a neutral अङ्ग (stem) अ , अम् is substituted for सु and अम् , the Nominative 1st-Case and Accusative -2nd case endings.
As कुण्डं तिष्ठति, कुण्डं पश्य ॥ So also वनम्, पीठम् ॥ Why do we not say "म् is substituted"? कुण्ड + म् = कुण्डाम्, thus there would be lengthening if only म् was taught 7.3.101, while कुण्ड + अम् = कुण्डम् (the single substitution of the form of the antecedent by 6.1.107.

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