According to the text, it can be stated that

It almost goes without saying that the native language of every learner is an extremely significant factor in the learning of a new language. Most of the time, we think of the native language as exercising an interfering effect on the target language, and indeed the most salient, observable effect does appear to be one of interference. The majority of learners’ errors in producing the second language, especially in the beginning levels, stem from the learner’s assumption that the target language operates like the native language. But what we observe may, like an iceberg, only be part of the reality. The facilitating effects of the native language are surely as powerful in the process, or more so, even though they are less observable. When the native French speaker who is learning English says, “I’m here since January”, there is one salient native language effect, a verb tense error stemming from French. But who is to say that the learner’s native French did not facilitate the production of that sentence’s subject-verb-complement word order, the placement of the locative (here), the one-to-one grammatical correspondence of every word in the sentence, rules governing prepositional phrases, and the cognate word (January)? In dealing with the native language effect in the classroom, interference will most often be the focus of your feedback in the classroom. That’s perfectly sound pedagogy. Learners’ errors stand out like the tips of icebergs giving us salient signals of an underlying system at work.

D. Brown. Teaching by Principles – An Interactive Approach to Language Pedagogy. Prentice Hall Regents, New Jersey. San Francisco State University. USA, 2007 (adapted).