The augment am (a) augment comes optionally after the ṛ of those roots which are exhibited in the dhātupāṭha as anudātta , when such roots end in a consonant having a ṛ has penultimate and are followed by an affix beginning with a mute or sibilant and not having an indicatory ka,
The word उपदेश is understood here, so also झल्यमकिति ॥ Thus त्रप्ता or तर्पिता or तर्प्ता or द्रप्ता, दर्पिता or दर्प्ता ॥ The roots तृप 'to satisfy', and दृप् 'to be happy to release', belong to Divadi class, sub-class Radhadi, and they take the augment इट् optionally 7.2.45, so we have the three forms given above : for these roots are also anudatta in their first enunciation.
Why do we say \which are anudatta in the Upadesa or the system of grammatical instruction\? Observe वर्ढा, वर्ढुम्, वर्ढव्यम्, from वृहू उद्यमने which is exhibited as उदात्त, and because it has an indicatory ऊ it takes इट् optionally. Why do we say 'having a penultimate ॠ letter'? Observe भेत्ता, छेत्ता ॥ Why do we say 'before an affix beginning with a mute or a sibilant'? Observe तर्पणम्, दर्पणम् ॥ Why do we say 'not having an indicatory क'? Observe तृप्तः, दृप्तः ॥ Before affixes not beginning with a झल् consonant this augment will not be inserted, as तर्पणम्, दर्पणम् ॥,