There are no official rules for "Standard English" because, unlike some other languages, English does not have a Language regulators such as the Académie française or Dansk Sprognævn to establish usage.
The English language, although originating in England, is now spoken as a first language in many countries of the world, each of which has developed one or more "national standards" of pronunciation, grammar, vocabulary, and spelling.
As the result of historical migrations of English-speaking populations and colonization, the predominant use of English as the international language of trade and commerce (lingua franca), English has also become the most widely used second language, and is therefore subject to alteration by non-native speakers. Numerous "non-native dialects" are developing their own standards those, for example, of English language publications published in countries where English is generally learned as a foreign language. In countries where English is either not a native language or is not widely spoken, another country's variant might be considered "standard", often that of England or the United States.
The article English grammar explains the complex grammar of Standard English. There are many grammatical variations in the many local dialects of English, but in formal written English and the "standard" dialects of English-speaking countries worldwide, the fundamental grammar is generally the same. There remain several open disputes in English grammar, often representing changes in usage over time.
With rare exceptions, national "standard" dialects use either American or British spellings, or a mixture of the two (such as in Canadian English). British spellings usually dominate in Commonwealth countries.
The definitions of words (such as "lift" vs. "elevator"), the grammatical properties of particular words (e.g. "in hospital" vs. "in the hospital" or "wait in line" vs "wait on line"), idioms, and slang vary considerably from country to country. With a few instances where confusion is possible (such as "pants", which means "trousers" in American English but "underwear" in British English), most vocabulary words are the same or mutually intelligible.
In the United States, General American is usually considered to be "standard" or "accentless", and is generally heard in the national media. In the United Kingdom, Received Pronunciation (RP) is sometimes considered "standard" or "proper", but many regional accents are heard on the British Broadcasting Corporation. Most countries adopt a variant of one of these accents or a local national accent as the "standard" pronunciation.
Some people consider local accents to be acceptable in formal contexts, but Trudgill believes that "Standard English is a dialect that differs from the others in that it has greater prestige, does not have an associated accent and does not form part of a dialect continuum".