A quick glance at a world map might suffice as explanation: the two countries are separated by the vast Atlantic ocean. The once same language stream was divided by this ocean and has remained separate since the colonialisation of Northern America by European immigrants.
The U.S is a vast country with many regional differences, where the various influences are more or less marked according to the history of the state. Louisiana, for example, was named by French settlers in honour of King Louis XIV, and uses many words borrowed from French in its local dialect, Cajun French: Andouille sausage and donut are local specialties, and fly a mielle replaces Bee. But above all, the words have all evolved in different directions since the colonialisation of the U.S by the English, in the same way that Quebec French has separated from France’s French. Two speakers from opposite sides of the Atlantic can still understand each other, but their manner of expression differs greatly.
Which version should you choose – British English or American English? Many students learn an essentially British English version of pronunciation and vocabulary, yet watch mainly American films and TV series. This leads to a lot of confusion both in written and spoken form. There is no reason to force an American over an English accent, as long as you don’t constantly flit between them, and it is precisely on this point that non-native English speakers struggle. Moving from one side of the Atlantic to the other can cause problems with spelling and vocabulary. It is interesting that British English tends to state more clearly than American English; an example of this is the difference in pronunciation of the words beer and bear. In British these would be bir and bair whereas in American these are virtually indistinguishable. Cat is kat in England and két in America. The usual rule is that the English use a form very close to the French, whereas Americans favour a spelling closer to the actual pronunciation of the word. For example colour/humour (UK) and color/humor (US) Theatre/centre (UK) and theater/center (US) and travelling/cancelling (UK) vs traveling/canceling (US) – this latter one will not help if you already have problems with double consonants... The vocabulary also differs greatly:
UK: lift, US:elevator
UK: sweet, US: candy
UK: flat, US: apartment
Things are complicated further when the same word has another meaning in the different countries: A biscuit is what you dip in your tea in England whereas in the US it is a kind of scone – they would obviously prefer to dip cookies. If you serve your guests potatoes, you can choose between chips (fries in the UK) crisps (chips in the UK) potato chips (chips, US) or fries (chips, US) or just drop the potato confusion all together and opt for a healthy salad. Finally a word of warning: you can tell an American that you like his pants (trousers) but avoid making this remark in London – you probably didn’t mean to compliment their underwear?