ऋदृशोऽङि गुणः

Operational rule vidhi

गुण is substituted for the vowel of the root before the affix अङ् in the roots ending in ऋ or ॠ as well as दृश् 'to see'.

Word by word (padaccheda)

Target ऋदृशः ṣaṣṭhī · ekavacanaRight context अङि saptamī · ekavacanaSubstitute / term prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
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

ऋदृशः अङि

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)

ऋदृशः → / अङि

Replace ṛdṛśaḥ with guṇaḥ when aṅi follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

गुण is substituted for the vowel of the root before the affix अङ् in the roots ending in ऋ or ॠ as well as दृश् 'to see'.
As शकलाङ्गुष्ठकोऽकरत्, अहं तेभ्योऽकरं नमः, असरत्, आरत्, अदर्शत्, अदर्शताम्, अदर्शन् ॥ The affix अङ् means the Aorist-character अङ् of which the above examples are given. It also is the krit-affix 3.3.104. of which we have जॄ--जरा ॥

The word अकरत् is अङ् Aorist formed from कृ by 3.1.59. The word असरत् is from सृ 3.1.56. अदर्शत् by 6.1.57.

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