सार्वधातुकमपित्

A सार्वधातुक -- तिङ्शित्सार्वधातुकम् 3.4.113, affix not having an indicatory ङित्।
The terminations which are added to roots are either Sarvadhatuka or ardhadhatuka. The Sarvadhatuka terminations are all those that are marked with a श् and also the tense terminations called तिङ्, the latter are the following:-

Parasmaipada

- 3rd Person. तिप् तस् झि
- 2nd Person. सिप् थस् थ
- Ist Person. मिप् वस् मस्

Atmanepada

- 3rd Person. त आताम् झ
- 2nd Person. थास् आथाम् ध्वम्
- Ist Person. इट् वहि महिङ्

Except the terminations marked with a प् such as ति, सि and मि, &c., all the other Sarvadhatuka terminations are ङित्. The शित् terminations like शतृ, शानच्, &c., are also Sarvadhatuka terminations. These affixes are also ङित्, unless they have a प् as indicatory, as शप्. Thus the affix as तस्, being an अपित् Sarvadhatuka affix, is a ङित्, and before it the vowel is not gunated e. g. कुरुतः 'they two do.' The equation is :- कृ + उ + तस् (the उ is added by Rule 3.1.79) = कर् + उ + तस् 6.4.110 = कुरु + तस् = कुरुतः ॥ Similarly कुर्वन्ति 'they do.'

Here the अ of कर् is changed into उ and we have कुर्, by virtue of rule 6.4.110, which declares that the अ of the verb कर् is changed into उ before those Sarvadhatuka affixes which are ङित् or कित्. Here तस् is a Sarvadhatuka ङित् affix by the present rule, and hence the change.

These four sutras describe the ङित् terminations.

Why do we say 'a Sarvadhatuka affix'? Because an ardhadhatuka affix, though not being marked with an indicatory प् will not be a ङित्. As कर्त्ता 'doer,' कर्तुम् 'to do,' कर्तव्यम् 'duty.'

Why do we say 'not having an indicatory प्'? If a Sarvadhatuka affix has an indicatory प् it will not be nit. As कृ + उ + तिप् = करोति 'he does' करोषि 'thou doest,' करोमि 'I do.'