यः सौ

Operational rule vidhi

The दकार of the word इदम् is converted to यकार when followed by the सुँ-प्रत्यय.

Word by word (padaccheda)

Substitute / term यः prathamā · ekavacanaRight context सौ saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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

यः सौ

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

Commentary

म् is substituted for the द् of इदम् before a विभक्ति affix.
As इयम् ॥ This is confined to the Feminine, as the following sutra relates to the masculine; and in the Neuter, the सु is लुक् elided by स्वमोर्नपुंसक and so leaves no trace behind.

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