ह्रस्वस्य गुणः

Operational rule vidhi

If an अङ्ग ends in ह्रस्व letter, that ह्रस्व letter gets a गुणादेश in presence of the सुँ-प्रत्यय of सम्बोधन एकवचन.

Word by word (padaccheda)

Target ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacana

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.

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

Full reading

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)

→

Replace hrasvasya with guṇaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the short इ and उ final in a nominal-stem, a गुण is substituted in the Vocative Case singular.
As हे अग्ने, हे वायो, हे पटो ॥ But there is no guna in हे कुमारि, हे ब्रह्मबन्धु, because ई and ऊ were shortened specifically, and to substitute guna for them would make their shortening a useless operation. Moreover, had guna been intended in the case of these nadi words, the sutra would have been अम्बार्थानां ह्रस्वः, २ नदीह्रस्वयोर्गुणः ॥

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