उपसर्गस्य घञ्यमनुष्ये बहुलम्

Operational rule vidhi

The final vowel of a preposition is diversely lengthened before a word formed by the कृत् affix घञ् but not when the compound denotes a human being.

Word by word (padaccheda)

Target ṣaṣṭhī · ekavacanaRight context घञि saptamī · ekavacanaRight context अमनुष्ये saptamī · ekavacanaSubstitute / term बहुलम् prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

घञि अमनुष्ये बहुलम्

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

As a rewrite rule (sketch from case endings)

→ बहुलम् / घञि अमनुष्ये

Replace upasargasya with bahulam when ghañi amanuṣye follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

The final vowel of a preposition is diversely lengthened before a word formed by the कृत् affix घञ् but not when the compound denotes a human being.
Thus नीक्लेदः, वीमार्गः, अपामार्गः ॥ It does not take place, as प्रसेचः, प्रसारः ॥ It is lengthened before साद and कार, when building is meant, as प्रासादः and प्राकारः, otherwise प्रसादः and प्रकारः ॥ Optionally in वेश &c. as प्रतिवेशः or प्रतीवेशः, प्रतिरोधः or प्रतीरोधः ॥ When human beings are meant, there is no lengthening, as निपादो मनुष्यः ॥

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