6.4.122
तॄफलभजत्रपश्च
Operational rule vidhi
ए is substituted for अ of 1. तॄ 2. फल 3. भज and 4. त्रप and the reduplicate is elided when the affixes of the लिट् (Perfect Tense) having an indicatory क् (or ङ्) follow, as well as when थल् of the लिट् (Perfect Tense) having the augment इट् follows.
Word by word (padaccheda)
Target तॄ- फल- भज- त्रपः ṣaṣṭhī · ekavacanaIndeclinable च avyaya
- Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
- Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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Full reading
तॄ-
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
ए is substituted for अ of 1. तॄ 2. फल 3. भज and 4. त्रप and the reduplicate is elided when the affixes of the लिट् (Perfect Tense) having an indicatory क् (or ङ्) follow, as well as when थल् of the लिट् (Perfect Tense) having the augment इट् follows.
Thus तेरतुः, तेरुः, तेरिथ, फेलतुः, फेलुः, फेलिथ, भेजतु, भेजुः, भेजिथ, त्रेपे, त्रेपते, त्रेपिरे ॥ In the case of तॄ the rule applies to the अ obtained by Guna, contrary to 6.4.126, in फल् and भज् the rule, that no different substitute should be in the reduplicate, is not adhered to, and in त्रप the अ does not stand between two simple consonants.
Vart:- It should be stated that the rule applies to श्रन्थ also:- as, श्रेथतुः, श्रेथुः ॥
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.