वा शोकष्यञ्रोगेषु

Operational rule vidhi

हृद् is optionally substituted for हृदय before the words शोक and रोग or the affix ष्यङ् follow.

Word by word (padaccheda)

Indeclinable वा avyayaRight context शोक- ष्यञ्- रोगेषु saptamī · bahuvacana

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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").

Full reading

वा शोक-ष्यञ्-रोगेषु

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

Commentary

हृद् is optionally substituted for हृदय before the words शोक and रोग or the affix ष्यङ् follow.
Thus हृच्छोकः or हृदयशोकः, सौहार्द्यम् or सौहृदय्यम् ॥ Here ष्यङ् is added as हृदय belongs to Brahmanadi class (V. I.I24). When हृद् is substituted there is Vriddhi of both the members सु and हृ by 7.3.19. So also हृद्रोगः or हृदयरोगः ॥ All these forms could have been got from हृत् which is a full word sui generis, having the same meaning as हृदय ॥ The substitution taught in this sutra is rather unnecessary.

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