To produce proficient speakers of English, we must offer correction in the classroom. The most obvious and oft-used form tends to be the direct teacher-to-student type, as in: "Akinori, you should say, 'Have you ever gone abroad?' instead of 'Have you ever went abroad?' Remember: go, went, gone." But this kind of correction proves the least desirable, especially if used often, because:
Teacher-to-student correction creates a teacher-centered classroom. In other words, students wait until after the activity to receive confirmation by you on a job well done (or not so well done, as the case may be).
Although there are a variety of techniques which encourage students to notice the language, and which you can employ throughout a lesson, correction from the teacher prevents students from noticing mistakes; and students need to notice their mistakes. It can also harm their ability to analyze why something is wrong.
It negatively affects confidence.
It lowers retention.