Every one of us, who isn't a native English speaker, started learning English very young. I remember my first years of learning English. I was in first class. The learning process was more connected with different games and just picking up vocabulary than seriously sitting down and learning the language. As time went by and the level of English became higher also the vocabulary and grammar became more difficult. Not only the level of English transformed but also the teachers changed from now and than. With every teacher also the kind of English differentiated. Some have prefered American and others British English. For me it was and still is really hart to keep track were both have differences. There are three major differences between American and British English.
- PRONUNCIATION
- SPELLING
- VOCABULARY
It doesn't matter if you prefer American or British English. You just need to be consistent and stick to one of them in your usage otherwise it will get really confusing. One important spelling differnece is that in American English words ending in -or and -er end in -our and -re in British English e.g. color/colour, center/centre. Another one is words ending in -ize in American English end in -ise in British English like realize/realise.
If you decide that you want to use Amercian English spellings then you can't write the following sentence The color of the apple is also his favourite. In this example sentence you can find both American and British English. Color is American spelling and favourite is British. This is of course not always easy but important to know.
My advice, if you are not sure about spelling and usage of vocabulary look it up in a dictionary. The dictionary can help you to be consistence that is also important for your language skills.