तासस्त्योर्लोपः

Operational rule vidhi

The सकार of तास् प्रत्यय and the verb root अस् is removed when followed by सकार.

Word by word (padaccheda)

Target तासस्त्योः ṣaṣṭhī · dvivacanaSubstitute / 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

तासस्त्योः

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)

तासस्त्योः →

Replace tāsastyoḥ with lopaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The final स् of तास् (the character of the second future) and अस् 'to be' is elided before an affix beginning with स् ।
As कर्त्तासि, कर्त्तासे, त्वमसि, ष्यति से ॥ See 6.4.111. In से the अ and स् both of अस् have been elided, the अ by 6.4.111, and स् by the present sutra,so that the mere suffix से remains, which however here is a finite verb: hence the स, is not changed to ष ॥ See 8.3.111.

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