The affixes called sat i.e. śatṛ and śānac are optionally the substitutes of lṛṭ (2nd Future).,
The सत् has been defined in 3.2.127. This option is a regulated option; that is to say, the affixes शतृ and शानच् replace लृट् under the conditions in which they replace लट्; thus by 3.2.124, these affixes will replace लृट् when agreeing with what does not end with the first case-affix. In these cases सत् will invariably replace लृट्. In other cases it will optionally replace लृट्. In these cases they will form what is known as future participles. As करिष्यन्तं देवदत्तं पश्य 'behold Devadatta about to do'; करिष्यमाणं देवदत्तं पश्य 'behold Devadatta being about to be done'. हे करिष्यन् or हे करिष्यमाण. Besides simple futurity, it denotes intention or purpose, as अर्जयिष्यमाणो वसति 'he dwells intending to earn his livelihood'; करिष्यमाणः सशरं शरासनं 'intending to furnish his bows with arrows'.
When, however, it agrees with the nominative case, it is optionally employed. As करिष्यन्, करिष्यमाणः, करिष्यति or करिष्यते देवदत्तः 'Devadatta will do'.
The form करिष्यन् is thus evolved. As कृ + लृट् = कृ + स्य + लृट् 3.1.33 = कर + इट् + स्य + शतृ = करिष्यत् nom. sing. करिष्यन् । In other words the participial affixes अत् (अन्त) and मान, technically known as शतृ and शानच्, also come after a verb that has taken the personal ending स्य of the Future.,