आतो ङितः

For the आ being a portion of a सार्वधातुक personal-ending which is ङित् (i.e. आते , आथे and आताम् and आथाम्), coming after a verbal अङ्ग (stem) ending in a short अ , there is substituted इय् ।
Thus पचेते, पचेथे, पचेताम्, पचेथाम्, यजेते, यजेथे, यजेथाम्, दास्यथे, दास्येते ॥ The य् of इय् drops by 6.1.66. Why do we say "the long आ"? Observe पचन्ति, यजन्ति, पचन्ते, यजन्ते ॥ Why do we say 'a ङित् affix'? Observe पचावहै, पचामहै 1.2.4. Why do we say 'ending in short अ'? Observe चिन्वति, सुन्वति ॥ Why 'short'? Observe मिमाते, मिमाथे ॥

All sarvadhatuka affixes which have not an indicatory प् are ङित् by 1.2.4 Now सार्वधातुकमपित् 1.2.4 is a sutra in which the word ङित् is understood from 1.2.1. But the grammatical construction of the word ङित् in the two sutras (I.2. I) and 1.2.4 is not the same. In the first sutra, it means ङिति इव - ङित्वत्, i. e. the roots गा कुट् &c are treated in the same way as they would have been treated, had a ङित् affix followed. But in the second sutra 1.2.4, the word ङिद्वत् is equal to ङित इव "like of ङित्" ॥ The sutra 1.2.1. is so explained in order to evolve the form उच्चुकुटिषति ॥ For when the सन् is added to कुट्, the root कुट् is treated as if the सन् was a ङित् affix, and so there is no guna. But if सन् itself had become ङित् then the Desiderative root चुकुटिष्, being a ङित् root would require atmanepada affixes by अनुदात्तङित आत्मनेपदम् (1.3. 12). In the case, however, of sarvadhatuka-apit-affixes, the affixes themselves become like ङित्, and are treated as ङित् affixes, not only with regard to the stem preceding them, but with regard to their own selves also, as we see in the present sutra.