प्रथमायाश्च द्विवचने भाषायाम्

Operational rule vidhi

आ is substituted for the final of युष्मद् and अस्मद् before the case-ending of the Nominative 1st-Case dual in the secular literature.

Word by word (padaccheda)

Target प्रथमायाः ṣaṣṭhī · ekavacanaIndeclinable च avyayaRight context saptamī · ekavacanaRight context भाषायाम् saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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").

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

Full reading

प्रथमायाः च भाषायाम्

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 Nominative 1st-Case dual in the secular literature.
As युवाम्, आवाम् ॥ Why of the nominative? Observe युवयोः, आवयोः ॥ Why in the Dual ? Observe त्वं, अहं, यूयं, वयं ॥ Why in the secular literature ? Observe
युवं वस्त्राणि पीवसा वसाथे in the Veda, so also आवम् ॥

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