Pāṇini sūtras

bhū is the substitute of the verb as ‘to be,’ when an ârdhadhâtuka affix is to be applied.

1. asteḥअस्तेः

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   aअ	astiअस्ति	asteḥअस्तेः	asअस्	astअस्त् + plus eḥएः

2. bhūḥभूः

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   bब्	bhūभू	bhūḥभूः	bhūभू	bhभ् + plus ūMऊM

Anuvṛtti

1. ārdhadhātukeआर्धधातुके 2·4·35

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   āआ	ārdhadhātuआर्धधातु	ārdhadhātukeआर्धधातुके	ārdhadhātukaआर्धधातुक	ārdhadhātukआर्धधातुक् + plus eए