Handling error in the classroom

Ignorance

the learner may simply have never learnt the form of the meaning and is just stabbing in the dark.  This is most common at lower levels because that's where learners' needs often outstrip their abilities to produce language.

Overgeneralising the rule

sometimes, when a 'rule' has been learned, learners will over-extend it.  For example, if you have learned the rule to add -ed to make a past tense, it seems logical to form catched.  Equally, over-extending a rule might lead to the production of wonderfuller.

First language interference

all learners, especially adult ones, will draw on language(s) they know to try to figure out a new one.  This is most obvious in the area of pronunciation, of course, but occurs frequently in other areas:
Structure: the learner's first language may have a structure that looks similar but means different things.  For example, the German structure of ich habe gesehen [I have seen, literally] often is better translated into English as I saw rather than I have seen.  That's only one reason for finding out a bit about our learners' first languages.
Lexis: many languages, and not only European ones, have words which look the same but have different meanings.  For example, simpatico in Italian means nice or friendly, sensibel in German means sensitive and un smoking in French is a dinner jacket.  There are many hundreds of these so-called false friends. 

Question 1: am I going to correct this?

If the error isn't holding up communication and has nothing to do with the subject of the lesson, it may be that correcting it will lead you off on a tangent, chasing red herrings and serve to slow down the lesson and confuse the learners.  If that's the case ignore it.
If, on the other hand, the error is impeding comprehension or is made in the language that is the target of what you are doing, then you will have to deal with it.

Question 2: do I need to correct this?

Very often, learners can correct their own production so a quizzical look or stopping learners and getting them to retrace their steps and reconsider may be more effective than your correcting the error.

Question 3: can anyone else correct the error?

If the learners can't correct their own errors, perhaps someone else in the class can.  If you think this is the case, give them the chance to do so.

Question 4: how will I correct?

A last resort is normally to give the correct answer yourself.  Often, learners can be led to discover the right answer with questions and suggestions such as "There's something wrong with the order of the words.", "What preposition do we need here?", "What tense should this be?", "Are we talking about tomorrow or today?", or "Is this a long 'a' sound or a short one?" and so on.
If all else fails, however, there are, obviously, times when providing the right answer is the best approach providing you make sure that the learner can produce the correct language independently after you have done so.

Now you have the basics, you can have a look at the slightly more comprehensive guide to handling error in the initial plus section of the site.  Much of what you have read here is repeated there so it should be simple to follow.

The next stage of this short course is concerned with moving forward, i.e., planning what to teach next (and after that).