Learning how to write, like learning how to function in a second language, can also be seen in developmental terms. Young writers must feel that they have ideas and a language system in their heads and that they can combine these to fill up blank sheets of paper. The final concern is whether or not the text conforms to the conventions of standard written English and is, therefore, correct. If the student is at a fairly advanced level and is making very few mistakes, then underlining or correcting all errors may be useful, especially if the paper in question is going to be rewritten. On the other hand, if a student is at a lower level, and has grammatical errors of one kind or another in virtually every sentence, then correcting all errors would mean a great deal of the teacher's time invested in a way more likely to result in student discouragement than improvement.
Students often tend to equate errors with failure.
With low and intermediate ESL students, I may add that at the end of the semester their English will still contain errors, but that their errors then will probably be more advanced and sophisticated than the ones they are producing now. For students who have not yet attained fluency and who are still struggling to express basic ideas or tell stories on paper in a second language, extensive writing in informal contexts such as journals will probably be more helpful than practice in revising and editing a small number of rough drafts into finished ones. The first question a teacher reading a paper by a low or low intermediate level second language writer should ask is whether the paper succeeds as a communicative act. A teacher who begins by taking the role of an interested audience intent on understanding the student's meaning, rather than the role of a critic of form, will still have comments to make about the formal aspects of language, because the process of communicating and understanding meaning sometimes demands attention to form. The question is not whether but when form should be the primary focus.