ईश्वरे तोसुन्कसुनौ

Operational rule vidhi

The affixes तोसुन् and कसुन् are added to roots in the छन्दस् (Vedas) to form Infinitives, when the word ईश्वर is in composition.

Word by word (padaccheda)

Right context ईश्वरे saptamī · ekavacanaSubstitute / term तोसुन्- कसुनौ prathamā · dvivacana

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").
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

ईश्वरे तोसुन्-कसुनौ

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

Commentary

The affixes तोसुन् and कसुन् are added to roots in the छन्दस् (Vedas) to form Infinitives, when the word ईश्वर is in composition.
Thus ईश्वरोऽभिचरितो = अभिचरितुम् । ईश्वरो विलिखः = विलिखितुम् । ईश्वरो वितृदः = वितर्दितुम् ।

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