Before the affixes 1. iṣṭha 2. iman and 3. īyas , r is substituted for ṛ in a aṅga (stem), when this ṛ 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 (V. I. 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 ॥,