When the relevant unit or structure of both languages is the same, linguistic interference can result in correct language production called positive transfer. However, language interference is most often discussed as a source of errors known as negative transfer. The interferential mistakes adopted into German from English as the first acquired foreign language are a new linguistic phenomenon among our students at the University of Economics in Prague. Frequent interlinguistic mistakes caused by mechanical adoption of the English language have been sorted out in several groups according to their lexical, grammatical and orthographical features, analyzed and described in detail. The output is a set of exercises which is published under the title "German after English". It is an exercise book with a set of interactive exercises on a CD-ROM. Our purpose is to eliminate common mistakes which make communication difficult. The most important aspect is the necessity of a comparative approach to teaching languages, which can help eliminate or avoid such mistakes.