Current practices and materials for teaching English as a second language (ESL) contain oversimplifications about English based on the assumption of a uniform language type and standard of usage, presented to foreign learners for pedagogical clarity. ESL teachers may be aware of the language's diversity but are not prepared to provide pedagogically and linguistically sound answers to questions about variations. English, with over 300 million native speakers around the world, is difficult to characterize accurately. Materials are available to teachers that discuss English varieties, including types labeled as "ex-colonial,""nativized" or "indigenized,""nuclear" or "utilitarian" for pedagogical and international usage purposes, regional and social dialects, jargons, slang, and stylistic variation. Choosing the appropriate functional style and switching freely from one to another is done automatically by native speakers on the basis of complex psychological and social clues that are part of cultural and linguistic experience. Foreign learners are often unable to perform or even comprehend these switches because their level of acquired English is functionally flat. ESL teachers should be ready to offer explicit advice and concrete examples about the global, regional, social, and situational dimensions of English, and plan strategies and design exercises directing students to a broader understanding of this diverse language. (MSE)