Before I ever became aware of language teaching or international English, there was a 1970's tv show called “Taxi”. One of Taxi's characters was an immigrant mechanic who desired to learn English but often made mistakes, both linguistically and culturally. Everyone loved him, though, partly because he was a nice person but also because even though his character was fictional, his language mistakes were real. In my high school, students would often quote some of his more famous lines, such as “I am pleased to meet me” and “Tank you bery much”. He was, for viewers, an opportunity to not have to be perfect all the time; he was their chance to not feel ashamed for making mistakes.
A few years later, while seeking accommodation near my university, I met a British lady who was renting out one of her spare bedrooms. While we talked, she told me a story about when she first arrived in Canada; she had invited a male friend to come by and “knock her up” in the morning. Now, in England to “knock someone up” means to come to their house and wake them up by knocking at the front door.
However, in North America, to “knock someone up” means, well, to create a serious problem for them; this problem could even result in a “shot-gun wedding” (there's another interesting idiom for you). When she told him this, he apparently choked on his tea and needed a moment to calm himself. She explained to him, though, what she was trying to say and they both had a good laugh about it.
Most ESL teachers are aware that making mistakes is important of and that it shouldn't be embarrassing. Especially when the mistakes are grammatical or lexical (using the wrong structure or the wrong word or phrase). Yet we still teach grammar as if the rules are absolute and inflexible. This is generally true until you start to consider that there are different dialects of English. Between each dialect there are subtle variations in the rules and the idioms. There may be two different ways to express an idea, and both are grammatically correct, but which one is used (or seldom used) can depend entirely on the region where English is spoken. There may also be some ideas or beliefs that exist in one area but not another; because of this, certain regions may not have the language necessary to communicate these concepts. Or the idiom may have two completely different meanings in two different regions.
For example, in my hometown we often refer to some women as being “skookum”. This is a word that was absorbed from the local indigenous (First Nations) language into our specific dialect, but it's a word that you won't hear anywhere else. To be “skookum” means that a woman is big or big-boned; it doesn't mean “fat” but rather that a woman is quite muscular. In the First Nations language, “skookum” has a positive connotation rather than a negative one. Another example is the word “bodega”, which was absorbed from Spanish into English (a bodega is a kind of grocery store). If you are moving to the USA, you would need to know this word in California or New York State, but you probably wouldn't need to know it in North Carolina.
So as teachers (or students who are taking responsibility for their learning), which grammar dialect do we teach or learn? Which sets of idioms and idiomatic language should we teach or learn? And should we introduce grammar rules as being absolute and inflexible? Internationally, most schools and teachers will teach either British English or American English, with Australian English being a not-so-distant third. To make it even more complicated, should we teach British English and identify the differences in American English (or vice-versa)? There isn't an easy answer to this, but I would suggest that we start by teaching the grammar rules (and vocabulary and spelling) in a general rather than absolute sense.
Students should be taught that this is how we generally communicate, that the grammar rules are generally true across all dialects. Allow the students to acquire the rules rather than just memorize them. Before moving further into the complexities between dialects, students should be able to subconsciously produce language using these rules; they may and probably will make some and perhaps many mistakes, but they will be able to use the grammar rule or item at least basically without having to consciously think about how and what to say.
Once they have acquired these language items, it will be easier for them to deal with the exceptions to rules and the differences between English dialects. In my opinion, assuming that they are normal, average language students, confronting them with the differences between dialects earlier on would be confusing and disheartening for them. It would simply be too much at once. Finally, teaching grammar and idiomatic language in a general sense makes it easier for students to understand that it's ok to make mistakes, since the rules can vary between dialects. Mistakes are what language learning is all about, in order to make it accessible and not overwhelming. Tank you bery much.