द्युद्रुभ्यां मः

Operational rule vidhi

The affix म comes in the sense of मतुप् after 1. द्यु and 2. द्रु .

Word by word (padaccheda)

Left context द्यु- द्रुभ्याम् pañcamī · dvivacanaSubstitute / term मः prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

द्यु-द्रुभ्याम् मः

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

Commentary

The affix म comes in the sense of मतुप् after 1. द्यु and 2. द्रु ।
Thus द्युमः and द्रुमः ॥ These are primitive words, and therefore do not take मतुप् in the alternative.

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