गन्धस्येदुत्पूतिसुसुरभिभ्यः

Operational rule vidhi

For the final of गन्ध is substituted इ when preceded by 1. उत् 2. पूति 3. सु and 4. सुरभि in a बहुव्रीहि समास .

Word by word (padaccheda)

Target गन्धस्य ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacanaLeft context उत्पूतिसुसुरभिभ्यः pañcamī · bahuvacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

गन्धस्य उत्पूतिसुसुरभिभ्यः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

गन्धस्य → / उत्पूतिसुसुरभिभ्यः

Replace gandhasya with it when it comes after utpūtisusurabhibhyaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the final of गन्ध is substituted इ when preceded by 1. उत् 2. पूति 3. सु and 4. सुरभि in a बहुव्रीहि समास ।
Thus the अ of गन्ध is replaced by इ, as उद्गन्धिः, पूतिगन्धिः, सुगन्धिः, सुरभिगन्धिः ॥ Why after these only? See तीव्रगन्धा वातः ॥

Vart:- The word गन्ध is a material noun as गन्धान् पिनष्टि, and an adjective as चन्दनगन्धः 'having the scent of sandal'. The rule of substitution applies when it is used as an attribute. Therefore, not here, शोभना गन्धा अस्य = सुगन्ध आपणिकः "a shop-keeper having many sweet perfumes for sale".

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.