From Kenneth Beare,
Your Guide to English as 2nd Language.

Differences Between American and British English

While there are certainly many more varieties of English, American and British English are the two varieties that are taught in most ESL/EFL programs. Generally, it is agreed that no one version is "correct" however, there are certainly preferences in use. The most important rule of thumb is to try to be consistent in your usage. If you decide that you want to use American English spellings then be consistent in your spelling (i.e. The color of the orange is also its flavour - color is American spelling and flavour is British), this is of course not always easy - or possible. The following guide is meant to point out the principal differences between these two varieties of English.

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Use of the Present Perfect

In British English the present perfect is used to express an action that has occurred in the recent past that has an effect on the present moment. For example:

I've lost my key. Can you help me look for it?
In American English the following is also possible:
I lost my key. Can you help me look for it?

In British English the above would be considered incorrect. However, both forms are generally accepted in standard American English. Other differences involving the use of the present perfect in British English and simple past in American English include already, just and yet.

British English:

I've just had lunch
I've already seen that film
Have you finished your homework yet?

American English:

I just had lunch OR I've just had lunch
I've already seen that film OR I already saw that film.
Have your finished your homework yet? OR Did you finish your homework yet?

Possession

There are two forms to express possession in English. Have or Have got

Do you have a car?
Have you got a car?
He hasn't got any friends.
He doesn't have any friends.
She has a beautiful new home.
She's got a beautiful new home.

While both forms are correct (and accepted in both British and American English), have got (have you got, he hasn't got, etc.) is generally the preferred form in British English while most speakers of American English employ the have (do you have, he doesn't have etc.)

The Verb Get

The past participle of the verb get is gotten in American English. Example He's gotten much better at playing tennis. British English - He's got much better at playing tennis.

Vocabulary

Probably the major differences between British and American English lies in the choice of vocabulary. Some words mean different things in the two varieties for example:

Mean: (American English - angry, bad humored, British English - not generous, tight fisted)

Rubber: (American English - condom, British English - tool used to erase pencil markings)

There are many more examples (too many for me to list here). If there is a difference in usage, your dictionary will note the different meanings in its definition of the term. Many vocabulary items are also used in one form and not in the other. One of the best examples of this is the terminology used for automobiles.

American English - hood British English - bonnet
American English - trunk British English - boot
American English - truck British English - lorry

Once again, your dictionary should list whether the term is used in British English or American English.

For a more complete list of the vocabulary differences between British and American English use this British vs. American English vocabulary tool.

Prepositions

There are also a few differences in preposition use including the following:

American English - on the weekend British English - at the weekend
American English - on a team British English - in a team
American English - please write me soon British English - please write to me soon

Past Simple/Past Participles

The following verbs have two acceptable forms of the past simple/past participle in both American and British English, however, the irregular form is generally more common in British English (the first form of the two) and the regular form is more common to American English.

Burn Burnt OR burned
Dream dreamt OR dreamed
Lean leant OR leaned
Learn learnt OR learned
Smell smelt OR smelled
Spell spelt OR spelled
Spill spilt OR spilled
Spoil spoilt OR spoiled

Spelling

Here are some general differences between British and American spellings:

Words ending in -or (American) -our (British) color, colour, humor, humour, flavor, flavour etc.
Words ending in -ize (American) -ise (British) recognize, recognise, patronize, patronise etc.

The best way to make sure that you are being consistent in your spelling is to use the spell check on your word processor (if you are using the computer of course) and choose which variety of English you would like. As you can see, there are really very few differences between standard British English and standard American English. However, the largest difference is probably that of the choice of vocabulary and pronunciation. For further information concerning these areas please refer to the following links below.

British vs. American English vocabulary tool
Check British to American English or American to British English with this interactive tool.

American-British/British-American Dictionary
An impressive resource for reference information concerning the differences in vocabulary use between American and British English.

American Vs. British English
An exhaustive study by the University of Tampere on the differences between American and British English and the geo-political reasons behind these differences.

English Around The World
An excllent link page to information, recordings, and vocabulary examples of English as it is used in many different countries around the world.

United Kingdom English for the American Novice
An amusing (some might find patronizing) instructive site dedicated to helping Americans understand United Kingdom English.