Considering the controversy about the nature of ergativity, the issue of whether a case morpheme is theta assigning or just structural case-marking, the present study tried to provide an evidence for either side or both. To this aim, the acquisition and use of ergative verbs were studied and possible explications of the errors were presented. According to the results, ergativity is not just a structural case, but a non-structural case marking. Besides, the uninterpretable features or feature trace can account for case errors and present potential explications for the agreement failure. Based on the principles and stages of the Contrastive Linguistics Hypothesis and Error Analysis, the results of the current study can be applicable for the practice of Teaching English as a Foreign Language (TEFL). After the linguistic description and explanation of the errors made by the Persian speakers, learning English as a foreign language, the TEFL practitioners can make use of the findings of the present study to see the points of difficulty and more focus on them.