2.4.52
अस्तेर्भूः
Operational rule vidhi
भू is the substitute of the verb अस् 'to be' when an आर्धधातुक affix is to be applied.
Word by word (padaccheda)
Target अस्तेः ṣaṣṭhī · ekavacanaSubstitute / term भूः prathamā · ekavacana
- Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
- Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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Full reading
अस्तेः भूः
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
अस्तेः → भूः
Replace asteḥ with bhūḥ. Inherited words (anuvṛtti) may add further conditions.
Commentary
भू is the substitute of the verb अस् 'to be' when an आर्धधातुक affix is to be applied.
भविता 'he will be,' भवितुम् 'in order to be' भवितव्यम् 'must be.' Now the Perfect tense affixes are also ardhadhatuka affixes 3.4.115. The Perfect of अस् is therefore the same as that of भू, namely, बभूव &c. There is, however, an important exception to this rule, namely, when अस् is employed as an auxiliary verb in forming Periphrastic Perfect of other verbs. In that case अस् forms its Perfect tense regularly, आस आसतुः आसुः &c. This apparent breach of the present sutra is explained by taking the term कृञ् in 3.1.40 as a Pratyahara including कृ, भू and अस्, and if अस् could not form its Perfect in the regular way, its inclusion in this Pratyahara would be useless; hence we conclude, that as an auxiliary verb, अस् forms its Perfect regularly.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.