The Imperative affix hi is elided after the u of an affix in the vikaraṇa with which the Present-aṅga (stem) (special conjugation) is made, provided that the u is not preceded by a conjunct consonant.,
The affix उ with which the Present-stem is made is उ and श्नु ॥ Thus चिनु, सुनु, कुरु ॥ Why after 'उ'? Observe लुनीहि, सुनीहि ॥ Why do we say \उ being part of the vikarana or affix\? Observe रुहि, युहि, here उ is part of the root itself, and not of the vikarana. Why de we say 'not preceded by a conjunct consonant'? Observe प्राप्नुहि, राध्नुहि, तक्ष्णुहि ॥
Vart:- The elision of हि after the उ of the vikarana उ and श्नु is optional in the Vedas; as आतनुहि यातुधानान्, धिनुहि यज्ञपतिम्, तेन मा भागिनं कृणुहि ॥,