सुपि च

Operational rule vidhi

The last letter of an अदन्त अङ्ग becomes दीर्घ when followed by a यञादि सुप्-प्रत्यय.

Word by word (padaccheda)

Right context saptamī · ekavacanaIndeclinable च avyaya

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

च

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

Commentary

Before a case-ending beginning with य् or भ् (literally a consonant of the यञ् प्रत्यहार) the final of a Nominal-stem is lengthened.
The whole of the phrase अतो दीर्घो यञि is understood here. Thus, वृक्षाय, प्लक्षाय, वृक्षाभ्याम्, प्लक्षाभ्याम् ॥ But अग्निभ्याम् where the stem ends in इ, and वृक्षस्य प्लक्षस्य where the affix does not begin with a यञ् consonant.

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