अतो हेः

Operational rule vidhi

The 'हि' प्रत्यय that follows an अदन्त अङ्ग is removed.

Word by word (padaccheda)

Left context अतः pañcamī · ekavacanaTarget हेः ṣaṣṭhī · 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).

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

Full reading

अतः हेः

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

Commentary

The Imperative affix हि is elided after a अङ्ग (stem) ending in short अ।
Thus पच, पठ, गच्छ, धाव ॥ But युहि, रुहि after stems ending in उ. Why do we say "short अ"? Observe लुनीहि, पुनीहि (लुना + हि, the ई substitution by 6.4.113, should be considered asiddha), here हि is not elided after the long आ ॥

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