शमामष्टानां दीर्घः श्यनि

Operational rule vidhi

Before the Present characteristic श्यन् , a long is substituted for the root-vowel in शम् and the seven roots that follow it.

Word by word (padaccheda)

Target शमाम् ṣaṣṭhī · bahuvacanaTarget अष्टानाम् ṣaṣṭhī · bahuvacanaSubstitute / term prathamā · ekavacanaRight context श्यनि saptamī · 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.
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

शमाम् अष्टानाम् श्यनि

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)

शमाम् अष्टानाम् → / श्यनि

Replace śamām aṣṭānām with dīrghaḥ when śyani follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

Before the Present characteristic श्यन् , a long is substituted for the root-vowel in शम् and the seven roots that follow it.
An शाम्यति, ताम्यति, दाम्यति, श्राम्यति, भ्राम्यति, क्षाम्यति क्लाम्यति and माद्यति ॥ Why of these eight only? Observe अस्यति ॥ Why before श्यन् ? Observe भ्रमति; the श्यन् being optional after this root by 3.1.70.

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