One of our wonderful group members had a question as to what makes British English and American English different. I couldn't fit my answer into the limited text box area so I chose to make it a discussion topic.
This is a great question! There are a lot of differences between American English and British English.
The first difference is the most ovious: pronunciation. The -er sound in British is almost never pronounced like the American rrrrr; rather, it would be more pronounced like uhhhh. For example: father. Americans say fah-thrrr and Brits (roughly) say fah-thuhhh. There are more examples, but I want to try to be as concise here as possible.
Another difference is the vocabulary. There are several terms that mean the same thing but use different words to express the idea. For example: loo and restroom both mean "toilet"; when in England, you ask where the 'loo' is, but, in America, you ask where the restroom is (literally, the room in which you use the toilet). Another example is swimming costume and bathing suit (or swimsuit); both mean the article of clothing that a person wears in order to go swimming. In England, you say swimming costume; but in America, you would never say this and would, instead, use the term swimsuit. Lastly, Britain's "smart pants" versus America's "dress pants." Both mean a pair of pants that are highly styled for the office or a professional appearance. (In all honesty, most Americans wouldn't understand the British term and may take offense by it, confusing it with "smartY pants" - which is someone who is annoyingly showing off their intelligence.
The last area that I'll discuss for today is punctuation and spelling. Let's talk spelling first, for example: "labour vs. labor" and "specialise vs. speacialize". If you spelled these words as indicated in the wrong countries, people - especially in academia - might think of you as uneducated because you "don't know how to spell correctly." "Labour" is British, while "labor" is American; likewise for the other spelling words. If you are trying to go to school or work for one of these two countries, you should write like that country - American for America and British for England.
Which leads me to punctuation: commas, hyphens, and even quotation marks are used differently. When listing things, for example: eggs, macaroni, and cheese. In England, most of the time the second comma would not be used - yet in America it would be. Therefore, the rules of punctuation differ from American to British English. Out of everything that makes the two systems of English different, punctuation is the least of your concerns; but, it is still good to know, nonetheless.
I hope this answers your question in some ways. There is a major difference between the two, and I do my best to make sure that my students understand this. Saying "Jolly good!" in America is never a good idea, especially for a man, unless you don't mind either being laughed at or scorned; the same goes for saying a vulgar quip like "f$cking-A" in England, as it is said in America.
So, depending on where a learner of English wishes to go in life, it is better for them to learn the English system of the country in which they want to be. That way, there is no confusion! If, however, you may need both systems, then study both - as I have encouraged my own students; just be open to their differences and especially their contradictory explanations, rules, and nuances.
Thanks for asking such a wonderful question!!
Bre, what do you think, whether learning mixed English, both American and British, without paying much attention to the differences, affects adversely to gaining success in any of them?