तनादिभ्यस्तथासोः

Operational rule vidhi

After the verbs तन् 'to expand' and others there is optionally the लुक् elision of सिच् when the आत्मनेपद affixes त and थास् of the 2nd Person follow.

Word by word (padaccheda)

Left context तन्- आदिभ्यः pañcamī · bahuvacanaRight context त- थासोः saptamī · dvivacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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").

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Full reading

तन्-आदिभ्यः त-थासोः

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

Commentary

After the verbs तन् 'to expand' and others there is optionally the लुक् elision of सिच् when the आत्मनेपद affixes त and थास् of the 2nd Person follow.
Thus अतत 6.4.37 or अतनिष्ठ 'he expanded,' अतथाः or अतनिष्ठाः 'thou didst expand.' Similarly असात or असनिष्ट 'he gave.' असाथाः or असनिष्टाः 'thou didst give.'

The roots तन् and सन् must both belong to the Atmanepada, and not to Parasmaipada. Hence in the Parasmaipada there is no elision of सिव्. Thus, अतनिष्ट यूयम् 'you did expand.'

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