The guṇa and vṛddhi substitutions which otherwise would have presented themselves do not however come, in the place of the vowels of didhi 'to shine' and vevi 'to go', and of the augment called iṭ ,
The roots दीधी and वेवी never take the guna or vriddhi substitutions under ordinary circumstances. Thus the affix ण्वुल् generally causes vriddhi substitution when added to any root; and so the affix ल्युट् causes guņa. But these affixes when applied to the verbs didhi and vevi, never cause vřiddhi or guna substitution. As आदीधी+ण्वुल् = आदीधी+अक 7.1.1 = आदीध्वकः 6.1.77. So also आदीधी+ल्युट् = आदीधी+अन 7.1.1 = आदीध्वनम्. Similarly from वेवी we have आवेव्यनम् and आवेव्यकः. In the Vedas, however, these verbs take guna. Thus अदीधेत् (Rig Ved. X.98.7), and अदीधयुः (Rig Ved. V.40.5). These two verbs are confined generally to the Vedic literature, and the present rule therefore, seems to be an unnecessary prohibition; as they take guna in spite of such prohibition.
Of the augment इट् the real affix is इ. In the general tenses, it is added to certain terminations beginning with consonants of the वल् class, i.e., all consonants except य 7.2.35. Thus the future termination, first person, singular is स्यामि; this when added to the root चर् \to move\ requires an intermediate इ, and thus we have the form चरिष्यामि, 'I shall walk.' This इ is never gunated or vriddhied, though according to the general rule, coming before an ardhadhâtuka termination, it ought to have been gunated.
The augment इट्, could under no circumstances have taken vriddhi, the prohibition therefore, is in the case of the guna substitute of इट्.,