ह्रस्वो नपुंसके प्रातिपदिकस्य

The short vowel is substituted in the neuter, for the final vowel, of a प्रातिपदिक।
In the neuter gender, the pratipadika that would have ended in a long vowel, changes it into short. Thus अति + रै = अतिरि 'extravagant', as अतिरि = कुलम् 'an extravagant family'; अति लो = अतिलु 'very hairy', अतिलुं कुलं 'a very hairy family.'

There is no substitution of short vowel, if the noun is not in the neuter gender, as ग्राम + नी = ग्रामणी, nom case ग्रामणीः 'the leader of a village or head-borough.' Similarly सेनानीः 'the leader of an army.'

A word may be in the neuter gender, but if it is not a pratipadika, the vowel is not shortened. As काण्डे तिष्ठतः 'two cantos remain.' Here the ए of काण्डे is formed by the coalescence of अ (of the pratipadika काण्ड) and इ (the dual case termination); thus काण्ड + इ = काण्डे. Now it might be urged that here one letter ए replaces two letters अ and इ. Therefore this substitute is equivalent to each of those letters separately, and consequently ए is the vowel of the pratipadika and ought to be shortened. But this is not to be so, because the word pratipadika has been especially mentioned in the text, and therefore this ekadesa is not treated as the final of the form which precedes it.