सप्तम्यां जनेर्डः

Operational rule vidhi

The affix ड comes after the verb जन् 'to be produced', with a past signification, when the word, in composition with it is in the Locative -7th-case.

Word by word (padaccheda)

Right context सप्तम्याम् saptamī · ekavacanaLeft context जनेः pañcamī · ekavacanaSubstitute / term ः 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").
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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

सप्तम्याम् जनेः डः

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

Commentary

The affix ड comes after the verb जन् 'to be produced', with a past signification, when the word, in composition with it is in the Locative -7th-case.
As उपसरे जातः = उपसरजः । जन् + ड = ज् + अ 6.4.143 = ज 'born of the first conception'; मन्दुरजः 'born in the stable'.

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