Recasting is when another person repeats a learner’s utterance, but with any errors in that utterance corrected. So if a learner says “the girl have a long hair” the other person (the interlocutor, who may be a teacher but can be another, more proficient learner) would say “oh…the girl has long hair”.
It is a way of focusing on form within a communicative situation, but its effectiveness seems to be highly disputed.
Doughty and Varela wanted to find out the effectiveness of using teacher recasts. They found that recasting significantly improved the accuracy of a form in oral use over the long term (in this study the form was reference to past time) BUT it was less effective in written use. (This may have been because the recasts were oral.)
e.g. if a child said: “I think that the worm will go under the soil“, the teacher’s recast would be: “You thought that the worm would go under the soil?”
Recasting seems both frequent and effective in child natural and classroom L1 and L2 language acquisition.
Doughty and Varela in Doughty and Williams, Focus on Form in Classroom SLA page 118
However, it has also been argued that recasts are more effective for adults than they are for children. It has also been argued that in a meaning-based lesson, children tend to ignore feedback on form because they are focused on meaning and simply see the recast as a continuation of the conversation.