आतो लोप इटि च

Operational rule vidhi

The final आ of a root is elided before an आर्धधातुक affix with the augment इट् as well as when it begins with a vowel and has an indicatory क् or ङ्.

Word by word (padaccheda)

Target आतः ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacanaRight context इटि saptamī · ekavacanaIndeclinable च avyaya

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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").
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

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

आतः इटि च

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)

आतः → / इटि

Replace ātaḥ with lopaḥ when iṭi follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

The final आ of a root is elided before an आर्धधातुक affix with the augment इट् as well as when it begins with a vowel and has an indicatory क् or ङ्।
Thus पपिथ and तस्थिथ ॥ Here the affix has the इट् augment. पपतुः, पपुः, तस्थतुः, तस्थुः ॥ Here the affixes are कित् by 1.2.5 गोदः, कम्बलदः with the affix क 3.2.3. So also प्रदा (fem), प्रधा (fem) by अङ् 3.3.106. Before Sarvadhatuka affixes, we have यान्ति, वान्ति, व्यत्यरे and व्यत्यले ॥ The two letter are the Imperfect Ist Person Singular Atmanepada of रा and ला with the affix इ (इट्). When it does not begin with a vowel, we have ग्लायते, दासीय ॥

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