After the verb śad 'to decay', when it has one of the affixes with an indicatory ś (śit) the ātmanepada is used.,
The root 'sad' when taking any affix which is marked with an indicatory श, is conjugated in the atmanepadi. In connection with this must be read sutra 7.3.78.पाघ्राध्मास्थाम्नादाण्दृश्यर्तिसर्तिशदसदांपिबजिघ्रधमतिष्ठमनयच्छपश्यर्च्छधौशीयसीदाः by which the root शद् is replaced by शीय before the affixes having an indicatory श. Thus शद् + श + ते = शीय + अते = शीयते 'he decays or withers,' शीयेते 'they two decay,' शीयन्ते 'they decay.'
Why do we say before affixes having an indicatory श? Before other affixes it is not atmanepadi. As अशत्स्यत् 'if he decayed,' शत्स्यति 'he will decay,' शिशत्सति.
The well-known vikaranas like शप्, श &c., the affixes like शतृ &c., are शित् affixes. In other words the root sad is atmanepadi in all conjugational tenses.,