By far the most difficult issue for speakers of several languages—such as Polish or Japanese—is the use of indefinite articles like “a” and “an,” or the definite article “the.” Those languages do not have articles, which is a problem, as they are the most common words in the English language.
Articles are a subset of the group of words that modify nouns called determiners. The use of determiners is sophisticated and complex, and there are many rules that describe when they should be used, which should be used, and what nuances of meaning they convey. I meant to write a whole series of tips about their proper use, but then I came across an excellent page that beat me to it: