ढे लोपोऽकद्र्वाः

Operational rule vidhi

The last letter of an उवर्णान्त-भसंज्ञक word is removed when followed by a ढकारादि-तद्धितप्रत्यय. But this does not happen for the word 'कद्रू'.

Word by word (padaccheda)

Right context ढे saptamī · ekavacanaSubstitute / term prathamā · ekavacanaTarget अकद्र्वाः ṣaṣṭhī · ekavacana

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").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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

ढे अकद्र्वाः

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)

अकद्र्वाः → / ढे

Replace akadrvāḥ with lopaḥ when ḍhe follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

For उ of ऊ of a भ अङ्ग (stem) is elided before the affix एय (ढ) but not of the अङ्ग (stem) कट्रू
Thus कामण्डलेयः, शैतिवाहेयः, जाम्बेयः, माद्रवाहेयः, but काट्रवेयो मन्त्रमपश्यत् ॥

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