Regardless of whether linguistic shortcomings are classed as mistakes or errors in accordance with Corder and James, the teacher also needs to consider the underlying cause of them. This is likely to be one of the following:

• inadequate knowledge of a rule (eg not knowing that Russian requires the genitive for animate direct objects);
• a rule wrongly applied (eg getting the order of French object pronouns wrong);
• over-generalization of a rule (eg putting the verb to the end of all German subordinate clauses);
• interference from L1 (especially useful when students share the same mother tongue); this can be one of the most productive ways to focus FL learners' attention on form (see Module 3).

Knowing the cause of deviance will help you to decide how to deal with it in feedback.