All language tutors will be aware that learners frequently produce incorrect forms when they in fact know the correct ones. It is the tutor's task to distinguish 'errors', the result of deficient knowledge, from such 'mistakes' which are merely occasional slips or lapses (Corder, 1967). For a detailed discussion of the nature of error and a review of the latest writing on the topic, see James (1998). Here you will also find the following classification of 'linguistic deviance', which represents a helpful refinement of Corder's distinction:

• Slips/lapses: these can be readily found and corrected by students themselves (eg when checking through a piece of work).
• First-order mistakes: these can be corrected by students themselves if they are prompted, or alerted to some shortcoming in their output (eg a mispronounced word is repeated back to them by the teacher).
• Second-order mistakes: these can be corrected by students themselves if they are told where they have gone wrong and what the nature of the problem is (eg if a word is underlined and 'subjunctive needed' is written in the margin).
• Errors: these cannot be corrected by students until they have been given further information and further learning has taken place.

(Based on James, 1998: 83)

Many of the errors that learners make do not resemble either FL or L1 forms. They are also remarkably regular and systematic, and it has therefore been suggested that they must be part of an internal learner system. This 'new' kind of language used by learners is often referred to as 'interlanguage' (Selinker, 1972). Interlanguage denotes an autonomous language system which is dependent on its own rules. The system changes constantly as learners progress in the language: for example, learners apply a new rule in one context and then gradually learn to apply it in another.

A significant feature of interlanguage is its variability (Tarone, 1988). At any stage of their development, learners sometimes use one form and sometimes another; this may involve alternation of one correct and one incorrect form, or even of two incorrect forms - probably the greatest source of tutor despair! Yet the choice of these forms is not entirely arbitrary: after some initial free variation, which may be a necessary stage in acquiring grammatical structures, learners start to use variable forms systematically, gradually improving these forms until they are correct. This systematic variability applies to the development of particular grammatical structures; for example, learners may have mastered the French imperfect tense but their use of 'de' and 'à' following adjectives may still vary freely.

Research suggests that errors are an inevitable and natural part of the language acquisition process. Making mistakes and errors, and learning from them, is an essential element of language learning, which is in essence a process of learners forming hypotheses about target language forms, refining these in the light of evidence from target language usage and employing their reformulated hypotheses to seek further input and extend their knowledge of the FL.

Furthermore, structures are not acquired in a linear, uniform or 'building-block' approach, despite what many textbooks and the layout of pedagogical grammars seem to suggest. Certain key structures are subject to an acquisition order and although this process can be hastened through formal work on the language and through targeted feedback, it will always be a gradual, incremental process, with fits and starts, forward spurts and intermittent regression. When you have corrected an aberrant auxiliary verb for the tenth time in a particular student's essay, maybe you can console yourself with this thought and tell yourself your breath and ink are not really wasted: he or she will get there eventually!

However, it is also well known that many learners never progress to target-language use in certain grammatical structures (Ellis 1997: 29). You can detect the beginnings of this 'fossilization' in advanced learners, in particular. For example, final-year undergraduates still produce basic errors in sentence structure or verb endings - despite many years of tuition; and some older learners, who have been attending advanced classes for years, still make basic errors and have poor pronunciation. In such cases, you need to give serious consideration to how to maintain students' motivation. If, for instance, after a lot of practice and correction a student's pronunciation does not seem to improve, you may want to concentrate on other aspects, such as stress and intonation, rather than try to improve pronunciation.