One of the most obvious functions of the teacher is to correct what the student says when he makes a mistake. This, however, is not so evident as it at first appears. There can be good reasons for not correcting some mistakes and even for "correcting" some usages which are not mistakes.
Curiously enough, NOT correcting is as important as correcting and is probably more difficult to do effectively. It's easy to come up with the right sentence after your student has put his foot in it and then to wait until he repeats what you said. This, however is not usually the most effective way to treat a mistake so, first, a few observations on the alternatives.
Correction should always be kept to a minimum and students should be made to correct themselves as far as possible so always try to find out if a student can put his own errors right before leaping in with "the right answer". With a new student you may, at first, have to say "No.", "Excuse me.", "Try again." or some such formula to encourage auto-correction, but if you are clear and systematic in your signals he will very quickly learn to react to a raised eyebrow or some such sign that what he's just said needs reformulating.
There is a big difference between the correction of an error and the reformulation of an answer because its original version isn't what the teacher wants and sometimes this can lead to confusion. If you say:
you will want a long answer from your student to find out if he can correctly place "last" in the sentence. However if you say "No. try again" or use any negative form to get him to reformulate his response, he is likely to think that "Last Tuesday" is not a valid answer to your question and, in an extreme case to panic and say "Three" or something like that which will confuse the situation irretrievably. On the contrary, just to emphasise the fact that he is on the right lines, it is probably best, to preface your intervention with "Yes" and say, for example:
starting off his sentence for him and making some sort of gesture to indicate that you want him to go on from there.
A student who is in the "student initiative" part of a lesson (when a certain autonomy is expected) should be expressing himself and his ideas at some length with a minimum of correction. Only mistakes which render the content of his summary, presentation or whatever incomprehensible should be corrected so as not to break the flow (though he can, of course note some misusages for treatment later). It is important that a student get a feeling of competence when he makes an effort to speak at length and constant intervention can be demotivating.
With fairly advanced students the problem can often be to decide which mistakes are "worth" correcting. Mistakes which are simply a question of grammar and of little consequence as far as comprehension of the listener is concerned are in a different category to mistakes which embody a real danger of misunderstanding. Ideally the student should have the potential danger of this latter kind of mistake drawn to his attention in a more striking manner than simply correction by the teacher. This can often be done by simply taking what the student says literally and then reacting to it in a logical way, creating a misunderstanding which forces the student to rethink and reformulate. A basic problem here for experienced teachers is to really understand the mistake when it crops up. When a French student says:
it is almost automatic for a competent teacher to move into some sort of correction procedure without considering for a minute that what the student said is in fact English but means something else if you take it at face value. The student can be pushed into a realisation that what he has said is misleading if the teacher continues along the following lines:
A lot of "false friends" can be treated this way too. French students often think "important" means "big" because it does in French.
"Profitable" is often confused with "advantageous", "beneficial", "good for something ":
The simple past is often used instead of the past continuous:
Listen, not only to what your student says but also to what it means, what it should mean, or what it could mean. This is what people he talks to will do when he gets out there in the big wide English speaking world and tries to survive with the tools you have given him. Sometimes the results he gets will be unexpected and he will have to deal with the situation. Give him some practice at it. Sometimes everything will seem perfect but there may be a residual misunderstanding. Let him see how this kind of thing can happen.