Articles in Spanish
There are two kinds of articles, both in Spanish and in English. In English, we have a definite article (the) that we use for nouns that are identifiable by the listener. For example, if I tell Mary I was at the restaurant last night, I'm assuming Mary knows what restaurant I am talking about.
We also have two indefinite articles (a/some) that we use for nouns that are not identifiable by the listener. If I tell Mary I was at a restaurant last night, it could be any restaurant, not a particular one that she already knows about.
Here are the articles in Spanish. They have to match the noun both in gender (masculine/feminine) and number (singular, i.e. one/plural, i.e. more than one).
Definite article (the)
el (singular, masculine) la (singular, feminine) los (plural, masculine) las (plural, feminine)
Examples: El piano, los pianos, la guitarra, las guitarras.
Indefinite article (a/some)
un (singular, masculine) una (singular, feminine) unos (plural, masculine) unas (plural, feminine)
Examples: Un piano, unos pianos, una guitarra, unas guitarras.
Articles work similarly in English and in Spanish. There is one exception, though: in English, generic/uncountable nouns at the beginning of a sentence don't have an article:
Cats are fun. Gas is expensive.
In Spanish, generic/uncountable nouns at the beginning of a sentence must have an article:
Los gatos son divertidos. La gasolina es cara.