इष्ठस्य यिट् च

Operational rule vidhi

After बहु , the augment यिट् is added to इष्ठ and भू is the substitute of बहु.

Word by word (padaccheda)

Target इष्ठस्य ṣaṣṭhī · ekavacanaSubstitute / term यिट् prathamā · 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.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

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

Full reading

इष्ठस्य यिट् 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)

इष्ठस्य → यिट्

Replace iṣṭhasya with yiṭ. Inherited words (anuvṛtti) may add further conditions.

Commentary

After बहु , the augment यिट् is added to इष्ठ and भू is the substitute of बहु।
As भूयिष्ठः ॥ This augment यिट् debars the lopa substitution, of the last sutra, in the case of इष्ठ ॥ The इ in यिट् is for the sake of pronunciative, the augment being य ॥ Or इष्ठ may be taken to have lost its इ by the foregoing sutra, and to the ष्ठ, the augment यि may be added.

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