Mistakes are common in the language learning process, so the role of the teacher is to correct them. To correct adequately and be aware to decide suitable strategies and techniques to do it. Mistakes can be assessed depending some aspects such as age, level, purposes and so on. Regardless the kind of mistakes, teachers are crucial guide to explain and improve this area.
Reference:
Spratt, M., Pulverness, A., & Williams, M. (2012). The TKT Course Modules 1,2 and 3 (2nd ed.). New York: Cambridge University Press.