एकाच्च प्राचाम्

Operational rule vidhi

After the word एक , according to the opinion of Eastern Grammarians, may be added the affixes डतरच् and डतमच् in the above senses.

Word by word (padaccheda)

Left context एकात् pañcamī · ekavacanaIndeclinable च avyayaTarget प्राचाम् ṣaṣṭhī · bahuvacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

एकात् च प्राचाम्

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

Commentary

After the word एक , according to the opinion of Eastern Grammarians, may be added the affixes डतरच् and डतमच् in the above senses.
The च draws in डतरच् ॥ That is डतरच् comes when one out of two is to be specified; and डतमच् when one out of many is to be specified. The word जातिपरिप्रश्ने does not govern this sutra: it being a general rule. Thus एकतरो भवतोर्देवदत्तः, एकतमो भवतां देवदत्तः ॥

The word प्राचाम् is for pujartha; the word वा being already understood in the sutra.

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