In order to determine interlingual morpho-syntactic L1-interference among 9 adult German native speaking subjects who attempted to learn the English language, long-term classes of an average timespan of 18 months were observed and tracked in regard of their most common errors in L2- syntax. During that period, it became obvious, that a majority of syntax errors occured due to interference with familiar structures of the subjects’ L1, which they had falsely mapped onto L2-structures. This paper describes the methodology of assessments of the most error-prone syntax structures in the featured pair of languages, the quality of subjectively-felt difficulties in syntactic switching as well as the specific associated fields of morpho- syntactic interference. Furthermore, it opens a follow-up discussion about whether root-similarities between L1 and L2 in general, in spite of their numerous advantages for learners, can also have negative impact on L2- acquisition in terms of syntax-errors due to the likelihood of ill-fitting mapping-attempts which might be brought forth by the faulty preassumption of identicalness, built upon the various similarities among L1 and L2.