फल्गुनीप्रोष्ठपदानां च नक्षत्रे

Operational rule vidhi

And the dual of फाल्गुनी and प्रष्ठपदा , when signifying asterisms, (also connotes optionally plural).

Word by word (padaccheda)

Target फल्गुनी- प्रोष्ठपदानाम् ṣaṣṭhī · bahuvacanaIndeclinable च avyayaRight context नक्षत्रे 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").

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Full reading

फल्गुनी-प्रोष्ठपदानाम् च नक्षत्रे

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

Commentary

And the dual of फाल्गुनी and प्रष्ठपदा , when signifying asterisms, (also connotes optionally plural).
The word च in this aphorism draws in the anuvritti of the word द्वयः from the last. Of the stars phalguni and proshthapada, the forms may be either dual or plural. As उदिते पूर्वे फल्गुन्यौ (dual); or उदिताः पूर्वाः फल्गुन्यः (plural.) So also पूर्वे प्रोष्ठपदे or पूर्वाः प्रोष्ठपदः ॥ There are two pairs of stars of the name of phalguni and proshthapada, both being of feminine gender, and their logical number being 'dual.' The present sutra ordains plural optionally. If phalguni and proshthapada are not names of asterisms, they must have their proper number : as फल्गुन्यौ माणविके ॥

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