एतेर्लिङि

Operational rule vidhi

The short is substituted for the vowel of the अङ्ग (stem) इ (एति) before the augment यास् in the Benedictive, when a preposition precedes it.

Word by word (padaccheda)

Target एतेः ṣaṣṭhī · ekavacanaRight context लिङि saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").

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Full reading

एतेः लिङि

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

Commentary

The short is substituted for the vowel of the अङ्ग (stem) इ (एति) before the augment यास् in the Benedictive, when a preposition precedes it.
As उदियात्, समियात्, अन्वियात् ॥ This is an exception to the following sutra by which a long would have been substituted. But ईयात् without a Preposition. The अण् 7.4.13 is understood here also. Therefore ए form of इ will not be shortened, as आ + इयात् = एयात्, समेयात् ॥

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