A, An & The

1. The equivalent articles in Spanish are un/una ("a, an", masculino/femenino), el/la ("the", masculino/femenino).
2. The use of the articles a and an depends on the coming phoneme, which is a phonological issue in English, not in Spanish.
3. These articles, like pronouns, demonstratives, adjectives and adverbs, have to reach an agreement in gender with the noun.
4. Ïn addition, el/la (=the) have to reach an agreement in quantity with the noun, their plurals are las/los.
5. Examples of other quantifiers are possessive personal pronoun, muchos/muchas (=a lot of), unos/unas (=some), un poco (=a little), todo (=all), nada (=none), alguno(=any) and numerals (un 1, dos 2, tres 3, etc...)
6. Spanish preposition a means "to" in English. When the article el is preceded by the preposition a, the words are combined into one word as al.
7. Example of [6]: "Vamos a el bar." (means "Go to the bar") becomes "Vamos al bar."
8. In Spanish, a preposition a is needed before a name of a person as an object, direct or indirect, in a sentence.
9. Examples of [8]: Veo a Rosa = (I) see Rosa. The preposition a is not needed if rosa is meant to be a flower. In the latter, an article is needed and the sentence becomes Veo la(=the) rosa(the letter r is not capitalised).

Reference:

1. 何仕凡, Español Básico, 世界圖書出版公司 2003