Of course, mistakes made bu students is a good symptom of learning something, but uncorrected are pretty dangerous as they tend to fossilize over time, which is no good :)
The question is... How to correct students in such a way that they know there is a mistake and they immediately feel a strong desire :) to correct themselves as only such a way seems so natural?
Here are a few tips:
- Don't accept ill structured sentences, instead reward good answers!
- Make an I-don't-get-it face and wait for the student to correct himself/herself.
- Ask the rest of the group to spot the mistake and give you the correct answer!
- Refer to some unit in a coursebook where you talked about a particular grammatical or lexical problem and make students find the right response.
- Choose the best student in your group and ask her/him if he knows how to correct his/her friends utterance.
- Pose a general question to the class and ask for the correct way of saying something.
- Instead of making a strange face you can ask a direct question: "Sorry, but I don't understand you. I think there is a mistake in your sentence/question".
- Accept the incorrect utterance but repeat it in the correct way stressing the erroneous area.