सार्वधातुके यक्

Operational rule vidhi

In the कर्मणि and the प्रयोगे भावे प्रयोग, धातु gets 'यक्' प्रत्यय when followed by a सार्वधातुक प्रत्यय.

Word by word (padaccheda)

Right context saptamī · ekavacanaSubstitute / term य prathamā · ekavacana

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

यक्

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

Commentary

The affix यक् comes after a root when a सार्वधातुक -- तिङ्शित्सार्वधातुकम् 3.4.113, follows, denoting the action or the object.
In forming Impersonal and Passive verbs, this affix is employed in the conjugational tenses. Thus in Impersonal verbs आस्यते भवता 'you sit', शय्यते भवता 'you lie'. So in passive verbs; as क्रियते कटः and गम्यते ग्रामः. The क of यक् is servile, and prevents guna and vriddhi. This affix is also used in reflective voice (कर्म्मकर्त्तरि); as क्रियते कटः स्वयमेव 'the mat becomes made of itself'.

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