विभाषा कृवृषोः

Operational rule vidhi

After the verbs कृञ् 'to do' and वृष् 'to rain', the affix क्यप् is optionally used.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaTarget कृवृषोः ṣaṣṭhī · dvivacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

कृवृषोः

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)

कृवृषोः → optional

Replace kṛvṛṣoḥ with vibhāṣā, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

After the verbs कृञ् 'to do' and वृष् 'to rain', the affix क्यप् is optionally used.
The root कृ would have taken the affix ण्यत्, by 124, and the verb वृष् would necessarily have taken क्यप् by 110; the present sutra therefore declares an optional rule. Thus: कृ + क्यप् = कृत्यम् 'to be done'; or कृ + ण्यत् = कार्यम्. So also वृष्यम् or वर्ष्यम् 'stimulating'.

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