बहोर्लोपो भू च बहोः

Operational rule vidhi

The इ and ई of these affixes (इमन् and ईयस्) are elided after बहु , and for बहु is substituted भू.

Word by word (padaccheda)

Left context बहोः pañcamī · ekavacanaSubstitute / term prathamā · ekavacanaWord भू Indeclinable च avyayaTarget बहोः ṣaṣṭhī · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Word Case not marked in the source data.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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

Full reading

बहोः भू च बहोः

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

As a rewrite rule (sketch from case endings)

बहोः → / बहोः

Replace bahoḥ with lopaḥ when it comes after bahoḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The इ and ई of these affixes (इमन् and ईयस्) are elided after बहु , and for बहु is substituted भू।
Thus भूयान्, भूमा ॥ In the case of इष्ठ, the following rule 159 will apply. Under 1.1.54 and 1.1.67, बहोः being in the Ablative, the first letter of the succeeding term is elided, viz. इ or ई; in this case. बहु belongs to Prithvadi class and takes इमन् affix. 5.1.122 The repetition of बहोः is for the sake of pointing out the sthani, for which the word भू is to be substituted : otherwise भू would have replaced these affixes.

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