But I can say that new teachers should be aware of the problems that could confront them, and they should, if possible, have tools to deal with them when they encounter them. B. Randomization Most teachers would agree that most grammatical errors have as their source 'interference' from the native language. In other words, if we could study the student's native language long enough, we should find the source of most of the persistent language problems of any given student. And, for that reason, if we have taught enough students from one language background, we should see common errors among them as they struggle with English while still using some of the rules and forms of their native language. But that is not the case.
Some learners have unique problems.
Teachers encounter students whose errors are not traceable to any source in the native language, students who are, furthermore, not being influenced by a third language or any other apparent source. The source of the problem is most likely, in my opinion, that the student's perception of English is a unique variable that influences the student's grammar, or interlanguage, and that that perception in fact makes each student unique, since students make up rules according to what they feel they know. They set up their own system, and we don't always know the source of their rules. As a teacher I have changed, over the years, from seeing myself as trying to change students' awareness of forms and structure, to seeing myself as trying to change the students' awareness and mastery of their own systems, of which knowledge and awareness of forms and structures are key components. I feel that the new language teacher could benefit from the careful analysis of the learning patterns of different learners.