Before a sārvadhātuka affix with an indicatory k or ṅ , short u is substituted for the a if kṛ 'to make' (kar + u) when guṇa-ated.,
Thus कृ + उ + तस् = कर् + उ + तस् 7.3.84 = कुर् + उ + तस् = कुरुतः 6.4.110 The तस् is ङित् by 1.2.4. So also कुर्वन्ति, कुरु ॥ Why do we say 'before a Sarvadhatuka'? So that the rule may apply to what was once a Sarvadhatuka, though no longer existent. Thus कुरु where the personal ending हि is elided, but it leaves its effect behind. The त् in उत् shows that this उ is not to be gunated by 7.3.86. When the affix is not कित् or ङित् we have करोति, करोषि and करोमि 1.2.4.,