Errors are mistakes caused by a learner not knowing something. They can be compared with slips, which are caused by a learner being tired, nervous, excited or another temporary factor.
The learner says ‘I buyed a new book' because she does not know that buy has an irregular past form.
In the classroom
Errors are a key part of the learning process and give teachers essential - and reliable - information about where their learners are, and what needs to be done next. It is important to emphasise the positive role errors have and to involve learners in correction, for example by using correction codes, doing group correction of writing or by dealing with typical spoken errors as a class after a speaking activity.