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解析：It depends on the using of language situation and the purpose of language teaching.When the focus of teaching is on language form,and when the students are doing mechanical pattern drills in the classroom.the teacher should correct the students whenever errors are committed.But when the students are engaged in communication, and the focus is on getting meaning across,errors should not be tackled on the spot,because they would be too much concerned with language forms and would never develop oral ability.In the same way,there might be more correction of errors in written than in oral communication.