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