4.1.33 पत्युर्नो यज्ञसंयोगे

Word by word (padaccheda)

Target पत्युः ṣaṣṭhī · ekavacanaSubstitute / term नः prathamā · ekavacanaRight context यज्ञ- saptamī · 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.
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").

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Full reading

पत्युः नः यज्ञ-

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

Commentary

The substitute न् replaces the final इ of पति before the feminine affix ङीप् when the word so formed means 'a wife' who takes part in the sacrifices of her husband.
Thus पत्नि वाचं यच्छ ॥ The feminine पत्नी means legitimate wife, she who is entitled to take part in the sacrifices of her husband. The word यज्ञसंयोग is an Instrumental Tatpurusha compound and means "related through sacrifice" i. e who shares the fruits of and is entitled to participate in, the sacrifices of her husband. But when the sense is not that of 'wife', the feminine of पति will be पति as ग्रामस्य पतिरियं ब्राह्मणी 'this Brahmani is the head or mistress of this village'. If the word पत्नी means 'the wife who takes part in the sacrifices of her husband', it cannot be applied to the wife of a Sudra, for a Sudra cannot perform a sacrifice, a fortiori not his wife? The term पत्नी is, however, extended to the wife of a Sudra also, by analogy, as the marriage of a Sudra also takes place by offering sacrifice to Fire and making it a witness.

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