Here is the "mini-essay"! Feel free to skip down to the final paragraph (or even skip everything entirely!

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PLEASE GIVE A BRIEF SUMMARY OF WHAT YOU CONSIDER TO BE THE MOST IMPORTANT ISSUE IN LANGUAGE TEACHING.
Actually, the only thing that I feel I’d want to write about (and am in any position to) is perhaps not so much an “issue”, as a gust of (if not exactly fresh, then at least reinvigorating) air flowing through the sometimes stale world of ELT; I am talking, “of course”, of the enormous contributions that computerised corpora have made to the description of language.
I don’t mean to imply that teaching (or learning!) is not a complex business, or that many of the decisions that need to be taken every day are not made easier by adopting an established metalanguage (i.e. an underlying belief system that, given the current state of “theoretical” linguistics, is not possible to prove or disprove the value of - so why not use it in the meantime?); it is just that a lot of the theories and “debate” that has informed ELT up to now has not been of such immediate and obvious practical value as, say, what the COBUILD project has achieved (“A COBUILD Dictionary in the hand is worth two grammar books on the resources shelf” – Confucius

). I sincerely believe, no, KNOW from experience that real data can clarify points of grammar (and often reveals new or clearer ones) in the teacher’s mind at least, and suggest more natural and varied ways to structure learning tasks and activities than previous (necessarily partial) descriptions could, and than almost a priori methodology still tries to do; both grammar and methodology cannot compensate for, or replace (a lack of) real knowledge (I use “knowledge” to refer not only to what a teacher should know, but also to what students might need to learn).
That being said, I am not sure that corpus data should “drive” (i.e. be the raw material for) language learning (some people have suggested bringing largely unedited, KWIC/concordance lines into classes, so that students can “notice” and “hypothesise” about possible meanings) - I think it is more the teacher’s job to do most of the analysis, and find ways (i.e. design language-based, communicative activities or tasks) to somehow relay and make “comprehensible” for learners any major conclusions that can be drawn from the data (this will obviously be especially necessary to do if the learners are not willing to be as independent as it is sometimes assumed they need or want to be. Sometimes, I suspect the best way to meet learner needs [expectations?] might be by actually teaching something – by assuming the roles of expert/native speaker, authority and resource - or at least suggesting what is to be “learnt” after tasks have been attempted and/or completed). It all sounds like a lot of work for the teacher, but it is what I ideally would try to do myself; and who minds hard work if it is rewarding (and hopefully aids learning)?! I suppose what I am ultimately saying is that I have an issue with issues, at least of the kind often presented in initial teacher training – you know, those things that are easy and pat “answers” to the trainers’ rhetoric, rather than balanced and sophisticated wider-ranging accounts.
I therefore believe that researching not just grammar, but now also the wider lexicogrammar is something that all teachers should do if they have the time, because the process will suggest valuable new areas and ideas for learning, and help revitalise one’s teaching too; I take what I would call a “corpus-inspired” approach to supplementing existing practice (Michael McCarthy, quoting Tognini-Bonelli in his Spoken Language and Applied Linguistics [Cambridge University Press: Cambridge 1998. 22-23] would call it a “corpus-driven” approach, but this to me has the negative connotations one might associate with “data-driven” learning, above).
(I could have perhaps said something about the objections that have been raised about corpora and lexical approaches: that they seem to be implying only native-speaker norms of idiomatic speech are acceptable, and that constraining the scope of utterances in this way will stifle creativity, lead almost to cliché, denies the successes and value of “slot-fill” structure-plus-word approaches, and might not develop “competence” as well as these other approaches; and that they help perpetuate the hegemony of the English-speaking nations. The facts are, however, that grammar is only one of the components of communicative competence – appropriateness is important in production - and that in reception, natural language is probably better in the long run for “acquisition” [or “learning”, if you prefer a more cognitive and/or explicit approach], that all being well should in turn feed into and aid production; that including non-native norms into corpora will ironically probably increase, not decrease the size and complexity of the learning load, if the universals of English usage will be as difficult to collate, identify and [equally] describe to the satisfaction of all concerned as realists would have us believe [besides which, there is nothing stopping the aggrieved from compiling their own “[inter]nationalistic” corpora if they feel present ones are inadequate. Surely they are a good start, to give them due credit?]; and finally that, as Hunston points out, learners of, in comparison, say Japanese “would be less disconcerted by having access to a corpus of language produced by native speakers [of Japanese]”*, despite the fact that Japan is also a powerful country with the potential to dominate. If other languages can be learnt in spite of such “issues”, can’t English also be, for whatever value it might have as “just” a language, a product of a culture, even if that culture has created inequitable institutions of world order? It is very hard to divorce language totally from culture, and not all students would want this to be the case; even scientists need their metaphors! Applied linguists trying to better describe English are hardly working on an atomic bomb, and their pleas that they are not responsible for any resulting destruction are not so self-serving when one considers the constructive uses to which English [and all languages] can actually be put).
* Hunston, S.
Corpora in Applied Linguistics. Cambridge University Press: Cambridge 2002. 194.