The article presents theoretical aspects of the effect of the second language on the first language, and describes the bidirectional influence between them. The article found that the effect of the L2 on the L1 significantly differs from the effect of the L1 on the L2, since L2 is cognitive and pragmatic rather than syntactic or lexical. The beneficial effect of FL learning on the development of L1 skills is just a potentiality- not a necessity. Not all kinds of L2 learning lead to the development of multi-competence. This will be possible only if exposure to the L1 is maintained at an appropriate level, otherwise, attrition may occur in L1 use. Content of exposure to both languages is more important than the quantity of exposure.