6.4.161
र ऋतो हलादेर्लघोः
Operational rule vidhi
Before the affixes 1. इष्ठ 2. इमन् and 3. ईयस् , र् is substituted for ऋ in a अङ्ग (stem), when this ऋ is preceded by a consonant and is not prosodially long (on account of being followed by a double consonant).
Word by word (padaccheda)
Substitute / term रः prathamā · ekavacanaTarget ऋतः ṣaṣṭhī · ekavacanaTarget हलादेः ṣaṣṭhī · ekavacanaTarget लघोः ṣaṣṭhī · ekavacana
- 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
रः ऋतः हलादेः लघोः
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
ऋतः हलादेः लघोः → रः
Replace ṛtaḥ halādeḥ laghoḥ with raḥ. Inherited words (anuvṛtti) may add further conditions.
Commentary
Before the affixes 1. इष्ठ 2. इमन् and 3. ईयस् , र् is substituted for ऋ in a अङ्ग (stem), when this ऋ is preceded by a consonant and is not prosodially long (on account of being followed by a double consonant).
Karika
पृथुं मृदुं भृशं चैव कृशं च दृढमेव च ।
परिपूर्वं वृढं चैव षडेतान् रविधौ स्मरेत् ॥
Thus प्रथिष्ठः, प्रथीयान् and प्रथिमा from पृथु 5.1.122, म्रदिष्टः, म्रदीयान् and म्रदिमा ॥ Why do we say 'the ऋ'? Observe पटिष्ठः &c. Why do we say 'preceded by a consonant'? Observe ऋजिष्ठः, ऋजीयान् ॥ Why do we say 'when prosodially short'? Observe कृष्णिष्ठः, कृष्णीयान्, कृष्णिमा ॥ The following are the six words to which this rule applies and to none else:- पृथु, मृदु, भृश, कृश, दृढ, परिवृढ ॥ It therefore, does not apply to words like कृत, मातृ, भ्रातृ, &c, as कृतमाचष्टे = कृतयति, मातरमाचष्टे = मातयति, भ्रातरमाचष्टे = भ्रातयति ॥
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.