तनोतेर्विभाषा

Operational rule vidhi

the lengthened of the अङ्ग (stem) of तन् is optional before the Desiderative सन् when it does not take the augment इट्.

Word by word (padaccheda)

Target तनोतेः ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacana

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.

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Full reading

तनोतेः

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)

तनोतेः → optional

Replace tanoteḥ with vibhāṣā, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

the lengthened of the अङ्ग (stem) of तन् is optional before the Desiderative सन् when it does not take the augment इट्।
Thus तितांसति or तितंसति ॥ But in तितनिषति no alternative is allowed, as इट् is added to सन् by 7.2.49 Vartika: optionally.

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