After ra and da for the ta of the Participial suffix ta and tavat there is substituted na and the same substitution takes place also for the preceding da,
After र :- आस्तीर्णम्, विस्तीर्णम्, विशीर्णम्, निगीर्णम्, अवगूर्णम् ॥
After द :- भिन्नः, भिन्नवान्, छिन्नः, छिन्नवान् from भिद् and छिद् ॥
Why do we say \after र and द\? Observe कृतः, कृतवान् ॥ The word र here does not denote the common sound र, which would include ऋ also, but the consonant र ॥ But even if र् be taken a common sound-name including र् and ऋ, yet the न् change does not take place in कृत &c, because between त and the र्-sound, there intervenes vowel-sound इ, for ऋ is sounded not like pure र्, but र् + a vowel sound.
Why do we say \of the Participial suffix\? Observe कर्त्ता, हर्त्ता ॥
Why do we say \for the त\? Observe चरितम्, हरितम् ॥ Here the त of the Nishtha does not follow immediately after र्, the augment इट् intervenes.
Why do we say \of the preceding\? The succeeding द will not be changed. As भिन्नवद्भ्याम्, भिन्नवद्भिः ॥
In the word कार्तिः the descendant of कृतः the त of Nishtha is immediately preceded by र्, but no change has taken place, because the Vriddhi, by which कृ is changed to कार्, is Bahiranga and consequently asiddha, and for the purposes of न change, the र् so obtained is invalid.,