2.3.23
हेतौ
Operational rule vidhi
When the word denotes 'cause' it takes the 3rd case-affix.
Word by word (padaccheda)
Right context हेतौ saptamī · 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").
Full reading
हेतौ
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
When the word denotes 'cause' it takes the 3rd case-affix.
विद्यया यशः 'by learning there is produced fame.' धनेन कुलम् 'by wealth, family;' कन्यया शोकः 'by daughter there is grief.'
The word हेतु here is used in its popular sense and not the grammatical hetu 1.4.55. Any thing capable of accomplishing a desired object is called hetu.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.