एकाद्धो ध्यमुञन्यतरस्याम्

Operational rule vidhi

After the word एक the substitute comes optionally instead of धा.

Word by word (padaccheda)

Left context एकात् pañcamī · ekavacanaTarget धः ṣaṣṭhī · ekavacanaSubstitute / term ध्यमुञ् prathamā · ekavacanaRight context अन्यतरस्याम् saptamī · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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").

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

Full reading

एकात् धः ध्यमुञ् अन्यतरस्याम्

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)

धः → ध्यमुञ् / एकात् अन्यतरस्याम्

Replace dhaḥ with dhyamuñ when it comes after ekāt when anyatarasyām follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

After the word एक the substitute comes optionally instead of धा।
Thus एकधा राशिं कुरु or ऐकध्यं कुरु, एकधा भुङ्क्ते or ऐकध्यं भुङ्क्ते ॥ The repetition of धा in the sutra shows that the substitution takes place when the sense is that of विधा, as well as when it is that of अधिकरण विचाल ॥ Had धा not been repeated, the substitution would have come in the sense of अधिकरण विचाल only, as immediately preceding this sutra.

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