5.1.132
योपधाद्गुरूपोत्तमाद्वुञ्
Operational rule vidhi
The affix वुञ् comes in the sense of 'nature or action thereof' after a polysyllabic stem whose penultimate letter is य् and whose penultimate syllable is prosodially heavy.
Word by word (padaccheda)
Left context य- pañcamī · ekavacanaLeft context - उपोत्तमात् pañcamī · ekavacanaSubstitute / term वु prathamā · ekavacana
- Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
- Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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Full reading
य-
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
The affix वुञ् comes in the sense of 'nature or action thereof' after a polysyllabic stem whose penultimate letter is य् and whose penultimate syllable is prosodially heavy.
The word उपोत्तमं means the last syllable but one in a word of three syllables or more. That word whose penultimate syllable is गुरू 'heavy', is called गुरूपोत्तमम् : the योपधा means 'having penultimate य्'.
Thus रामणीयकम् from रमणीय, वासनीयकं from वसनीय ॥
Why do we say "the penultimate letter being य्"? Observe विमानत्वं from विमान ॥
Why do say गुरूपत्तिम "the penultimate syllable being heavy"? Witness क्षत्रियत्वम् ॥
Vart:- Optionally so after the word सहाय, as, साहायकम् and साहाय्यम् ॥
The words उपोत्तम् and उपधा should be distinguished: the first means penultimate syllable (implying thereby the word is of more than two syllables), and the second means penultimate letter.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.