In this paper, I have focused on the foundational and fundamental problems of the Turkish copula, i.e., to clarify its definition and functions. No matter how powerful the latest theory is, if the target of discussion is wrong, the result does not explain the language properly. To do this, I have looked at previous studies on the Turkish copula and seen that they have the many problems. Previous studies with respect to the copula have the following problems;1) The traditional school grammar: the list of grammatical morphemes has not made ​​up properly; ① They have not understood the fact that the so-called predicative verb is just the copula. ② Since the copula is an inflected word, prefinal and final or personal endings are added to its base. However, they have regarded personal endings as the copula. ③ They have considered the modality ending {-Dİr} as the third person singular ending of the so-called predicative verb and counted that both negative postposition {değil} and the {-Dİr} are the copula. ④ They have called the Turkish copula a verb or auxiliary verb. 2) In case of the copula is the focus of discussion: an agglutinative feature of the Turkish language has been ignored in their discussions and the theory of general linguistics and achievements of the study on the European languages were brashly applied to the Turkish copula; ① The copula, an inflected word, can be divided into its stem and suffixes. However, they have committed an error to equate the copula with personal endings in using the terminology of the complex categories such as <copula 1st singular>, <copular element>. ② They have seen the copula as a verb. ③ The semantic classification and hierarchy were excessively applied to the analysis of Turkish copular sentences. Finally, in this paper we have argued that the copula not only connects syntactically the predicate to the subject but also grammatical element to other ones