Even though we have learned a foreign language after many years of studying, it"s rare to see people speak the language fluently that they have learned. There must be some reason why this happens repeatedly to many people. In this paper we analyse the process of speaking or understanding a certain language from the point of linguistic theory, in order to compare the learning of foreign languages with the acquisition of mother tongues. From the results of this comparison, we suggest that the learning of foreign languages must be performed not by the aid of the mother tongue but directly by way of the languages in question. If the learning of foreign languages is done with the aid of the mother tongue, the students can"t reach the conceptual level where comprehension is operated, and it doesn"t help students develop communicative competence. In case that we attach importance to communicative competence, we must find occasions which let students use the expressions that they have learned. It also means that we must have suitable circumstances for the development of communicative competence.