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Given the ambiguous nature of the essay question (i.e., does 'anyone' include native or non-native speakers? Where does 'know English well' fit on the continuum of native fluency to non-native speakers who can get by? Should such 'knowledge' include functional usage, understanding of the mechanics and components of English etc., or the ability to accurately convey this knowledge?) this essay will address the following issues: a) which criteria define a competent teacher b) is 'native' fluency sufficient qualification to teach English or is proficiency and understanding of its structure required? c) are 'native' and 'non-native' speakers able to teach English equally effectively?
How we define 'teaching' influences, to some extent, our expectations of teachers e.g., as facilitators in learner-centred approaches or as models of correct usage in more traditional 'knowledge transmission' or 'chalk and talk' approaches.  But what of those language 'teachers' - often unqualified 'native' speakers - without professional training? Many inevitably fall into the role of 'entertainers' as, beyond modelling or providing exposure to correct usage (although, as we shall see, this is questionable), they are unable to provide systematic learning, or take on the role of assessor, tutor, prompter or resource etc., instead amusing the class with their antics which may 'cover up the fact that very little has been taken in and used by students.'  As Ur posits, teaching is a concept intrinsically and inseparably bound up with learning - while 'learning may take place without conscious teaching; but teachingâ€¦ is intended to result in personal learning for students, and if worthless if it does not do so.'  Thus the haphazard, unsystematic approach of an untrained teacher may result in learning but is hit-and-miss at best and not an effective use of learners' time and/or investment.
Other schools of thought claim that respect and empathy for students and teacher 'authenticity'  are more important teacher qualities than the practical techniques taught during formal training.  While the role of rapport in creating a learning atmosphere in which students feel able to be honest and take risks should not be underestimated, Scrivener argues that a teacher's performance is but a minor factor in conveying and sharing knowledge: although a positive learning environment provides a good foundation its role should not be exaggerated '[b]eing jokey, chatty and easy going doesn't necessarily lead to good teachingâ€¦ [as such] â€¦lessons can end in confusion.'  That is not to say that knowledge of subject matter and methodology are alone sufficient - while much can be learnt with these two, 'total learning' could be lacking without an aware, sensitive, and enthusiastic teacher who understands students and seeks to enable learning and create the conditions to encourage learning, not merely perform as a teacher. 
In light of such definitions, should teachers' pedagogical competence outweigh the import of their 'historical origin' i.e. native fluency? In the age of globalisation is the native/non-native dichotomy valid? According to Chomsky's definition, 'native' speakers are authorities or 'ideal informants' on their language  - a dubious premise as we shall see - yet it is often geography that is the main determinant of 'native' speaker status.  Should, for instance, teachers who acquire English as a first language - along with local languages - in countries outside Kachru's centre/inner circle countries be considered 'native' speakers or 'balanced bilinguals'?  Tellingly, they are often considered 'poor imitations' of native speakers, deficient teachers as 'continuous learners' of the target language. 
Yet the dichotomy creates a preference for 'native' speaker fluency over proficiency in English and professional ELT training - reflected in the advertising and hiring practices of language school worldwide - serving to undermine both the import of teacher training and teachers from the 'periphery'.
As the forces of globalisation intensify trans-cultural contact, socio-economic and technological change - in turn shaping migratory patterns - national boundaries and historic identities increasingly blur and merge - impacting significantly on the ELT industry and the English language. English, while long historically prominent due to British colonial expansion and more recent US hegemony,  is now faced with the prospect of becoming a 'true' international lingua franca as it enjoys hitherto unprecedented levels of international recognition and vitality.  While it is clear that this global gravitation towards English - although not toward one particular native variety otherwise regional and national varieties would be obsolete - is certain to continue to transform the language, predicting the long-term effects on both 'native' and 'non-native' speakers remains controversial. 
The practice of categorising varieties of English and, thus, 'native' and 'non-native' speakers (and teachers) is increasingly considered restrictive  in the modern era as the number of post-colonial 'non-native' varieties of English grow exponentially  and international migration increases. In ELT such classification, primarily focusing on ethnicity, arguably acts to negate formal education, linguistic expertise, professional teaching experience and preparation.
English's role as a/the official language in post-colonial regions, for instance, has resulted in generations of 'native' (at least in their own variety) English speakers - the language often becoming deeply entwined in such countries national identities and socio-cultural infrastructure, both being shaped by and shaping the overall evolution and spread of English. Yet such ex-colonial states claims of ownership of varieties of (historically/current official or national) colonial languages has sparked controversy as many consider such Englishes as mere 'illegitimate offspring'  - primarily on the biased basis of failing to follow 'native' usage. 
Indeed, this theoretical conception of a legitimate 'native' speaker arguably stems from a perception of colonial subjects as illiterate, primitive, incompetent speakers and inadequate learners - a received social prejudice adopted by linguistic schools of thought at the time - which was reinforced by colonists initial unwillingness to teach English to the colonised - their inadequate English was believed to create a necessary distance between the rulers and the ruled. 
In modern times there remains the discriminatory notion that 'native' speakers/teachers are superior to 'non-native 'teachers - while outnumbering native ELT teachers 4:1,  non-native teachers tend to have their qualifications, pronunciation and ability to teach effectively more closely scrutinised and, broadly, are overall marginalised in the ELT profession.  While exceptions exist in countries where demand for English teachers outstrips supply e.g., proficiency takes on a more prominent role in China and educational background in countries where English is a/the official national language, this overwhelming tendency to employ 'native' teachers - partly due to their social 'prestige' - remains the rule: their educational background and professional experience are not mandatory or of, at best, secondary importance. Indeed, this trend flies in the face of English's evolution i.e., its progressive detachment from historically native speaking countries, the diminished significance of accent as non-native outnumber 'native' speakers  - and recent research demonstrating that European non-native English speakers have a better command of the language than native-speakers. 
The effect of elevating the importance of 'native' speakers - regardless of their level of proficiency or professional ability - is to relegate non-native teachers (often to secondary or support roles) and diminish the standing of professional training and experience in ELT. Arguably, this oversimplified native/non-native classification system fails to take into account ability and, broadly, fails to question whether 'native' teachers - particularly the poorly qualified/experienced or unqualified and inexperienced variety - necessarily have more to offer than non-native qualified, experienced teachers or whether qualified, experienced native teachers are more effective than qualified, experienced non-native teachers.
Research into this area is inconclusive although a number of trends emerge: 'native' teachers are characterised broadly more informal and flexible in terms of methodology etc., better at using authentic English, conveying subtle nuances of meaning, offering accurate models of usage or engaging students in conversation, but less organised and professional than their non-native counterparts.  Non-native teachers are portrayed as good role models and English language 'information providers' who, while often overly reliant on textbooks and less able to offer spontaneous conversation, are able to contrast differences between L1 and L2 (often due to a shared mother tongue), prepare learners for exams more effectively than 'native' teachers, and offer effective learning strategies based on insights gleaned from their own study of English - having undergone the process of language acquisition arguably makes teachers better qualified to teach, anticipate learner difficulties and, drawing on their knowledge of students' backgrounds and the psychological aspects of language acquisition, be more sensitive to students' overall needs. 
Thus the case for qualified, experienced non-native teachers appears strong when compared to untrained (and, in some cases, qualified) 'native' speaker teachers - but what of 'native' speakers' (whether qualified or unqualified teachers) ability to use natural, authentic English with 'correct' stress and intonation patterns? While 'native' speaker teachers arguably provide more 'comprehensible input' and exposure to natural language, there is a danger that, if taking on the role of 'performer'- more likely among inexperienced, unqualified teachers - talking time is dominated by teachers.  It is debatable, however, whether all but the most experienced 'native' teachers are able to provide 'roughly tuned' language as well as a non-native teacher familiar with learners' mother tongue. With the majority of native teachers monolingual, it is doubtful whether untrained native teachers would be able to guide learners beyond elementary levels i.e., learning through 'immersion'/exposure to English or patterned drilling etc., can only go so far without explanation of the language's mechanics as 'raw, unmediated new input is often incomprehensible to learners; it does not function as intake, and therefore does not result in learning.' 
Perhaps the chief reason untrained native speakers remain in high demand is the widespread conviction that they speak a form of Standard English and are thus 'appropriate linguistic models for their students and colleagues.'  The perception of native speaker's infallibility - and long history of prominence in linguistics as a benchmark for correct language production and evaluation i.e. ability to provide accurate, reliable judgments on correct, authentic language usage and intuitively identify ill-formed sentences without necessarily being able to explain why  - is premised on the notion that native speakers acquire their language during childhood without the interference or influence of other languages.  The issue, however, is arguably less clear cut: the majority of 'native speakers' are native speakers of a non-standard variety of the language  i.e., putting them in a similar position to, for instance, non-native speakers for whom English is the official language: both groups must overcome similar hurdles (e.g., professional and social-cultural) to achieve ELT proficiency.
'Historical origin' is thus no guarantee of proficiency or infallibility although 'native' speakers may possess superior knowledge of socio-cultural variables that inform the content and delivery of utterances given their uninterrupted exposure to a specific English-speaking environment from infancy.  Indeed, linguistic error analysis emerged from the study of native speakers - suggesting to some that ELT based primarily on a native speaker's assumed competence - without formal training - is hardly a 'safe haven for error free language transmission'  due to variations in dialect and culture etc., among native speakers. Effective teaching arguably needs to incorporate knowledge of learners' socio-political and cultural realities in order to bridge communicative gaps etc., so mere 'native' speaker knowledge - however perfect - seems an insufficient and unnecessarily restrictive requirement or criterion for determining teachers' worth or ability.
Equally, social judgement and historical bias seem inadequate criteria for judging an English teacher's competence e.g., the 'native' speaker social prestige factor outlined above has arguably nullified the importance of teacher competence and proficiency, promoting instead a form of 'linguistic imperialism'  which seems to fly in the face of the realities of the contemporary globalised world and complex, multi-ethnic modern face of English. English Language teaching is a profession requiring professional training and not an inherent, innate facet of a native speaker's repertoire or abilities. Indeed, there is a strong case for reviewing the native/non-native dichotomy in line with present day realities to avoid elitism and marginalisation: the term 'native' is overly restrictive and imbibed with prejudice and historical bias.
To reiterate, the ELT profession and institutions arguably have a moral obligation to ensure students are not exposed to unqualified teachers and untrained native speakers should not be employed for both practical and pedagogical reasons: less committed to the profession, more costly than non-native teachers and typically monolingual - still regarded by some as an advantage i.e., students will spout R.P. if the teacher exclusively uses English during classes - which often means teachers are a) unable to clear up misunderstandings that arise, b) lack experience of learning a second language (thus lack empathy with learners) and of being bilingual and c) often means they are also mono-cultural. 
Anyone who knows English well is not equipped to teach it - an egalitarian standard should be devised that expands on TESOL's criteria  and takes in account the variation in 'native' standards and gives equal weight to teachers' competence, teaching skills, experience, professional preparation and proficiency.