ॠत इद्धातोः

Operational rule vidhi

The last letter of the दीर्घ-ॠकारान्त-धातु is converted to इकार.

Word by word (padaccheda)

Target ॠतः ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacanaTarget धातोः ṣaṣṭhī · 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.

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

Full reading

ॠतः धातोः

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)

ॠतः धातोः →

Replace ṝtaḥ dhātoḥ with it. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the final long ॠ of a root, there is substituted इर् -- उरण् रपरः 1.1.51.
As किरति, गिरति from कॄ and गॄ of the Tudadi class. आस्तीर्णम् विस्तीर्णम् from स्तॄ, the lengthening by 8.2.77. Why do we say of a root? Observe पितॄणाम्, मातॄणाम् ॥

This substitution will apply to Derivative roots also, as चिकीर्षति from कॄ 'to scatter'.

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