Whenever we propose to talk about this grammatical class now represented by verbs, we are, without any doubt, aware of its complexities. Among them, not to mention the main one, are the conjugations – represented by the different forms that the verb can assume from the moment it is inflected.
Thus, based on these inflections, the verbs are subdivided into three basic groups, demarcated respectively by the thematic vowels “-a” – “-e” – “-i”. Thus, in order to represent them, we quote the verbs sing, sell and leave which, when conjugated, always they obey a conventional model called paradigm, which indicates the verbal forms called regular.
So that we can effectively understand how this whole process actually materializes, let us combine these verbs, keeping in mind the present tense in an indicative way: 
It is not enough effort to verify that in any way there was a change in the radical, and the endings seem to follow a certain conventionalism, maintaining a correlation with the already expressed “vowel thematic”.
However, such standardization tends, in some specific circumstances, to deviate, corroborating so that the verbs acquire their own nomenclature, in view of the peculiar traits that feature. Among these, there are the so-called irregulars, whose main characteristic is demarcated exactly by irregularities with regard to the radical and/or endings.
Representing them, we have the case of verbs ending in “-iar”, in which we see these irregular features. Once present in primitive times, they tend to extend themselves into the corresponding derivatives as well. In a special way, we present the verbs to mediate and their respective derivatives (to mediate and to remedy), to yearn, to burn and to hate. But, after all, what would be the peculiar trait that defines them? The truth is that they all receive the intercalation of the vowel “e” in the rhizotonic forms.
Based on this assumption, we will check this occurrence more closely, choosing, for that, the verb to mediate:
Indicative mode
subjunctive mode
Imperative mode
Personal infinitive
Nominal forms
By Vânia Duarte
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/verbos-irregulares-terminados-iarparticularidades.htm
