The Spanish definite article has to agree in gender and number with the noun it modifies, and it doesn’t always correspond to an article in other languages.
The singular indefinite articles in Spanish correspond to “a, an, one” in English. The plurals correspond to “some.” There are four Spanish indefinite articles.
Spanish’s neuter article, lo, is invariable and can be used in front of just about any adjective in order to express something abstract or a quality.
There’s an interesting phenomenon in Spanish regarding feminine nouns that begin with a stressed A sound. When these nouns are singular and preceded directly by a definite article, the masculine article is used instead of the feminine article you might expect.