When we talk about mistakes when learning a second language we talk about an unconscious derivation when a learner sometimes uses one form and sometimes the other - he "gets it right" - but sometimes he makes a mistake and uses a wrong form. (GADUSOVÁ, Z. and HAR?ANSKÁ, J: Methodology of teaching English as a foreign language, Anthology of Methodological Text, 1994)

In other words a mistake is incorrect language which a native speaker would not usually produce, that is, sometimes that only learners of the language produce.

There are two sorts of mistakes:

-         The mistake that occurs when a speaker uses a correct piece of language (linguistic form) that does not mean what the speaker wanted to mean.

-         The mistake that occurs when the speaker uses a correct linguistic form that is socially unacceptable - the big problem here being one of politeness.

From a teacher´s opinion of how a mistake fits in with an individual student´s stage of learning in her class, mistakes can be divided up into different categories:

a)      Slips: If the teacher thinks that a student could self correct a mistake.

b)      Errors: If a student cannot self correct a mistake in his or her own language, but the teacher thinks that the class is familiar with the correct form.

c)      Attempts:  When the teacher knows that the students have not yet learned the language necessary to express what they want to say.

(EDGE, J: Mistakes and correction, Longman, 1989).