When the tense-affixes of the Perfect follow, there is reduplication of the root, which is not already reduplicated.,
The reduplication is either of the first syllable or of the second syllable according as the root begins with a consonant or a vowel. Thus पच् + णल् = पच् पच् + णल् = पपाच ॥ So also पपाठ, प्रोर्ण्णुनाव ॥ In the case of ऊर्णु, Rule 3.1.36 does not apply, so the Perfect of this word is not formed by adding आम् and the auxiliary verbs कृ, भू and अस्; but regularly. In fact ऊर्णु is regarded as if it was नु ॥ See sutra 3.1.36 Vartika. वाच्य ऊर्णोर्णुवद् भावो यङ् प्रसिद्धिः प्रयोजनं, आमश्च प्रतिषेधार्थमेकाचश्चेडुपग्रहात् ॥
Why do we say when लिट् follows? Observe कर्त्ता, हर्त्ता ॥ Why do we say \of a Dhatu\? Observe ससृवांसो विशृण्विरे, सोममिन्द्रायसुन्विरे ॥ The question arises from the fact that the root (dhatu) always directly precedes the affixes of the Perfect, no vikarana intervenes as in the case of other tenses. So there was no necessity of using the word dhatu in the sutra. To this it is replied that by 3.4.117, sometimes लिट् is treated as sarvadhatuka, and then it takes vikarana. As श्रु taking the vikarana श्नु becomes शृणु; 3.1.74 this whole base is not a dhatu, therefore when the affixes of Perfect are added there is no reduplication, and we have शृन्विरे ॥ See 3.4.117.
Why do we say 'of a non-reduplicate'? Observe कृष्णो नोनाव वृषभो यदीदम् । नोनूयतेर्नोनाव, i.e. it is the Perfect Tense of the Intensive verb नोनूय, and is not reduplicated again, यङ् having already caused reduplication. So also संमान्या मरुतः समिमिक्षुः ending in the उस् of the Perfect.
Vart:- In the Chhandas there is optionally reduplication of the root in the Perfect and other tenses. As आदित्यान् याचिषामहे or यियाचिषामहे; देवतानो दाति प्रियाणि or ददाति प्रियाणि ॥ मघवा दातु or ददातु, नस्तुतो वीरवद् धातु or दधातु ॥
Vart:- The root जागृ is optionally reduplicated in the Perfect. As, यो जागार (or जजागार) तमृचः कामयन्ते ॥,