रभेरशब्लिटोः

Operational rule vidhi

The verb root रभ् which is followed by an अजादि प्रत्यय gets a नुमागम. But this नुमागम is not seen in presence of शप् विकरणप्रत्यय as well as in presence of लिट्-लकार.

Word by word (padaccheda)

Target रभेः ṣaṣṭhī · ekavacanaRight context अ- शप्- लिटोः saptamī · dvivacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

रभेः अ-शप्-लिटोः

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

Commentary

The augment नुम् is added after the vowel of the root रभ् 'to begin' before an affix beginning with a vowel but not before the विकरण शप् or the affixes of the लिट् (Perfect Tense).
Thus आरम्भयति, आरम्भकः, साध्वारम्भी, आरम्भमारम्भम्, आरम्भो वर्तते ॥ But आरभते in शप्, and आरेभे in लिट्, and आरब्धा before an affix beginning with a consonant.

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