From Readers Digest:
The British founded America’s original thirteen colonies, so we should be speaking in the same dialect. Right?
. . . .
First, let’s go over a lesson in Linguistics 101. An accent is a varied pronunciation of a language. A dialect is a variety of a language that includes different vocabulary and grammar, in addition to pronunciation. Two important factors in the formation of a dialect are isolation from the source of the original language and exposure to other languages.
The “American English” we know and use today in an American accent first started out as an “England English” accent. According to a linguist at the Smithsonian, Americans began putting their own spin on English pronunciations just one generation after the colonists started arriving in the New World. An entire ocean away from their former homeland, they became increasingly isolated from “England English” speakers. They also came in more contact with foreign languages, those of the Native Americans and other settlers from Sweden, Spain, France, and the Netherlands. Both factors eventually led to changes in Americans’ vocabulary and grammar, creating a new English dialect. (However, there is some British slang that Americans don’t realize they use.)
An important reason why American English and British English sound different is rhotacism, the change of a particular sound in a language. In this case, that sound is “r.” The standard American accent—what Americans think of as having no accent—is rhotic, meaning that speakers pronounce their “r’s.” Received Pronunciation (aka typical British accents) is non-rhotic, so words like “card” are pronounced like “cahd.”
At first, English speakers in the colonies and England used a rhotic accent. But after the Revolutionary War, upper-class and upper-middle-class citizens in England began using non-rhotic speech as a way to show their social status. Eventually, this became standard for Received Pronunciation and spread throughout the country, affecting even the most popular British phrases. Americans kept their rhotic American accent—for the most part. Port cities on the East Coast, especially in New England, had a lot of contact with the R-less Brits. So if you always wondered why Boston natives pahk theyah cahs to pahty hahd with a glass of cabahnet, thank rhotacism. This is why Americans drive automatic and Europeans drive manual.
Link to the rest at Readers Digest