Abstract: The object of the research is interlanguage interference as one of a normal effect in teaching and learning foreign languages. The subject of the research is the specifics of this interference that can be used to teach Spanish to students who have already learnt English. The authors analyze the process of teaching the second foreign language as a pedagogical process which differs from the process of teaching the first foreign language fundamentally. The authors focus on the relationship between English and Spanish language systems and whether it is possible to develop a critical thinking based on teaching a foreign language. The research methods used by the authors included general research methods and special research methods. General research methods clarify the role of teaching the second foreign language in general, special research methods explain specifics of teaching Spanish based on English and interlanguage interference that arises in this process. The authors conclude that from the methodological point of view it is possible not only to minimize interlanguage interference but also develop a critical thinking on this basis which is very important in the era of 'instrumental' ideas in language education caused by pragmatic communicative concepts. Along with a detailed analysis of opportunities for working with linguistic materials based on a rarely taught language, this makes the novelty of this research.