मृजेर्वृद्धिः

Operational rule vidhi

The verb root मृज् undergoes a वृद्धि.

Word by word (padaccheda)

Target मृजेः ṣaṣṭhī · ekavacanaSubstitute / term 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

मृजेः

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)

मृजेः →

Replace mṛjeḥ with vṛddhiḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The वृद्धि (आर्) is substituted for the root vowel (ऋ) of the अङ्ग (stem) मृज् before an affix.
As मार्ष्टा, मार्ष्टुम्, मार्ष्टव्यम् ॥ The मृज् here is a root, and the affixes before which this Vriddhi takes place are those which come after roots, and not which come after Pratipadikas. Therefore, not before the affixes भ्यां &c, as कंसपरिमृड्भ्याम्, मृड्भिः ॥ The anuvritti of the word vibhakti has ceased. This sutra debars guna of 7.3.84.

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