This thesis aims at investigating occurrences of error types produced by L1 Hungarian learners of L2 English from the perspective of whether L1 transfer errors represent an outstanding figure among other categories and therefore serve as the most influential obstacles in the SLA process overtaking intralingual and developmental error types. Composing another interest area of the study, the hypothetical dominance of interlanguage, that is transfer errors raise the issue whether grammatically incorrect and semantically weird TL constructions can be, and if it is so, on what bases, attributed to two methods of L2 application: translating to the target language vs. thinking in the L2. The types of inappropriate grammatical structures in L2 were scrutinized among elementary school students progressing in the 8th grade and fitting to the requirements of the lower-intermediate level. Two bilingual and two conventional language learner groups were tested by 3 different tasks focusing on error inducement and error recognition which also intended to study the methods of SL usage formerly described here. The participants were also asked about facts regarding learning English even as which task(s) had proved to present difficulty. The test results as well as statistics calculated let it draw the conclusion that although such tests were prepared for participants that tried to eliminate the potential emergence of ungrammatical constructions resulting from transfer by their nature, the majority of errors generated by the learners compose L1 transfer errors irrespective of bilingual or conventional classes. Moreover, these results may be contributed to the assumption that the participants applied mere translation instead of formulating their thoughts in the TL.