नदी बन्धुनि

Operational rule vidhi

In a बहुव्रीहि समास having the word बन्धु as its second member, the first member ending in the feminine affix ई (नदी word) has the ac on its final syllable.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaRight context बन्धुनि saptamī · ekavacana

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

बन्धुनि

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

Commentary

In a बहुव्रीहि समास having the word बन्धु as its second member, the first member ending in the feminine affix ई (नदी word) has the ac on its final syllable.
Thus गार्गी꣡बन्धुः, वात्सी꣡बन्धुः ॥ The words गा꣡र्गी and वा꣡त्सी are formed by adding ङीष् 4.1.16 to गार्ग्य and वात्स्य ending in यञ् 4.1.105, and therefore, they are first acute. By 6.2.1 this accent would have been retained, but for the present sutra. Why do we say "a Nadi (Feminine in ई) word"? Observe ब्र꣡ह्मबन्धुः, the word ब्रह्म has acute on the first syllable as it is formed by मनिन् (Unadi IV. 146) and it retains that accent 6.2.1. Why do we say "before बन्धु"? Observe गा꣡र्गीप्रियः ॥

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