Many learners ask about the differences between British English and American English... but this thread has nothing to do with that.

There seems to be some confusion, even -- or especially -- amongst native speakers, as to whether or not English has dialects. I take "dialect" to mean a variety of a language which is associated with a particular geographical area, or social or ethnic group, and which can be distinguished from other varieties by differences in pronunciation ("accent"), vocabulary and grammar, but without those differences being great enough to substantially impede mutual intelligibility with other varieties of the language.

To simplify matters here, I'd like to just focus on the English of England.

The characteristic accents of different regions of England (e.g. the use of the vowel /ʊ/ in the north for southern /ʌ/, the rhoticity of the south-west, post-vocalic spirantisation and distinctive intonation of Scouse, L-vocalisation and th-fronting in east London, loss of /h/ and /t/ being replaced by /ʔ/ in many areas) are well known (at least in the British Isles); regional vocabulary (e.g. nesh, bairn, nowt, gang, settle, aye, anyroad, summat, baht, mardy) is not hard to come by either. But what about grammar? Well, there are differences in grammar within England too: there are areas where there is still a distinction between singular, informal thou and plural, formal you (with corresponding verb forms), which has been lost in "Standard" English (in this respect the dialectal practice is more similar to French, Welsh and many other European languages); in many areas some verbs are conjugated differently (e.g. the "Northern Subject Rule" -- a feature also found in, and probably borrowed from, Celtic languages); prepositions, pronouns, conjunctions, adverbs and other features may be used differently ("he were bigger nor her", "he did it hissen", "two on 'em", "tell I", "wam, norrit?"). So I would conclude that the diversity of the language in England is conspicuous enough to be able to say that English does have dialects, even though the dialectal divergence may not be as great as in some other languages, such as German or Italian.

But some people do deny the existence of dialects in English, and I suspect one reason may be that the dialects are generally not recognised as such in the popular mind, instead they are often just considered "bad English", as if they were merely corrupted forms of "proper English". I find the situation quite different to Italy, a country well known for the variety of its dialects (some of which have even been officially promoted to the level of "language"). There, although some people, especially in the past, would associate speaking in dialect with being uneducated, the dialects are, nonetheless, recognised as part of regional heritage and considered fundamentally distinct from the national language, Italian. In Italy there is also widespread diglossia, whereby people switch between Italian and "dialetto" according to the social situation and who they are talking to; I don't think this is so much the case in England, probably due to the first point above.

Feel free to add your own thoughts or to give some examples of dialectal English (examples from outside England are welcome too).