द्वितीयायां च

Operational rule vidhi

आ is substituted for the final of युष्मद् and अस्मद् before the case-ending of the Accusative -2nd case.

Word by word (padaccheda)

Right context द्वितीयायाम् saptamī · ekavacanaIndeclinable च avyaya

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").
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

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Full reading

द्वितीयायाम् च

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

Commentary

आ is substituted for the final of युष्मद् and अस्मद् before the case-ending of the Accusative -2nd case.
As त्वाम्, माम्, युवाम्, आवाम्, युष्मान् and अस्मान् ॥ This rule applies to endings which are even substitutes, otherwise the last rule would have been sufficient. See 7.2.28, 7.2.29, and 7.2.92 and 7.2.97.

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