अमो मश्

Operational rule vidhi

For the personal ending अम् of the 1st person singular (in the Aorist) म -- अनेकाल्शित्सर्वस्य (1.1.55) is substituted in the छन्दस् (Vedas).

Word by word (padaccheda)

Target अमः ṣaṣṭhī · ekavacanaSubstitute / term मश् prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

अमः मश्

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

As a rewrite rule (sketch from case endings)

अमः → मश्

Replace amaḥ with maś. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the personal ending अम् of the 1st person singular (in the Aorist) म -- अनेकाल्शित्सर्वस्य 1.1.55 is substituted in the छन्दस् (Vedas).
The अम् here is the substitution of मिप्, and not the accusative singular affix. As वर्धी वृत्रम् (for अवधिपम्) Rig I.165.8: क्रमीम् वृक्षस्य शाखाम् ॥ The अट् augment is diversely elided 6.4.75. The indicatory श् of मश् shows that the whole of the affix अम् is to be replaced. The substitution of म् for म् is to prevent the change of म् into anusvara as in 8.3.25.

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