A Comparative Grammar of the Modern Aryan Languages of India v3.c17.82

v3.c17.82

Here the only noteworthy feature is the change of ष्य into स्स. The Âtmanepada follows the same rule throughout. Although the tendency to keep that form of the root which exists in the present leads to divergences from the Sanskrit future type, yet instances occur in which the Sanskrit type is preserved. These occur in reference to that very troublesome feature in the Sanskrit verb, the intermediate इ, which is sometimes inserted between the root and the termination, and sometimes not. When it is not inserted, the euphonic laws of Sanskrit require that the final consonant of the root be changed to enable it to combine with the initial consonant of the termination. Thus √ पच् “cook,” when it has to take the future termination ष्यति, becomes पक् and पक् + ष्यति = पक्ष्यति. Here Pali sticks to the form पच्, because it is used in the present and makes its future पचिस्सति as though there had been (as there probably was in colloquial usage) a Sanskrit future पचिष्यति with the intermediate इ inserted.