पूर्वं तु भाषायाम्

Operational rule vidhi

In the common speech, the end vowel of the first alternative is only प्लुत.

Word by word (padaccheda)

Substitute / term पूर्वम् prathamā · ekavacanaIndeclinable तु avyayaRight context भाषायाम् saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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

पूर्वम् तु भाषायाम्

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

Commentary

In the common speech, the end vowel of the first alternative is only प्लुत।
This makes a niyama or restriction. The previous sutra, is thus confined to Vedic literature, and not to the vernacular. तु here has the force of 'only'. Thus अहि र्तू꣡ ३ रज्जुर्नु 'Is it a snake or a rope'? लोष्टो तू꣡ ३ कपोतोनु 'Is it a clod of earth or a pigeon ?'

The priority, of course, depends upon employment. The order of words depends upon one's choice.

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