सान्तमहतः संयोगस्य

Operational rule vidhi

The स्वर preceding the नकार from (a) महत् and (b) any word that ends in न्स् - becomes दीर्घ in presence of प्रत्यय that is also a सर्वनामस्थान, other than सम्बोधन-एकवचन.

Word by word (padaccheda)

Word सान्त Target महतः ṣaṣṭhī · ekavacanaTarget ṣaṣṭhī · ekavacana

Word Case not marked in the source data.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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

Full reading

सान्त महतः

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

Commentary

In the strong cases with the exception of Vocative Case singular, the penultimate vowel is lengthened in the case of a अङ्ग (stem) ending in स् with a Nasal consonant preceding it, and of महत्।
That is, a stem ending in the conjunct consonant न्स्, elongates its penultimate vowel before the affixes of the first five cases. Thus श्रेयान्, श्रेयांसौ, श्रेयांसः, श्रेयांसि, पयांसि, यशांसि ॥ महत्-महान्, महान्तौ, महान्तः ॥ But हे श्रेयन्, हे महन् in Vocative Singular. The Nasal is inserted by 7.1.70.

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