Which Article Should You Use?
Let’s take the noun apple and see how our choice of definite and indefinite article will shade our perception of the word.
An article either points out a noun as a specific member of its class, as if pointing to it with one’s finger, in which case the definite article is used. Thus, the apple refers to a specific apple, not just any old apple. Perhaps the one the speaker is even pointing at it. Now, let’s suppose the speaker knows that there is a bag of apples in the refrigerator and doesn’t care which one he eats, he might ask someone to bring him an apple – hence the indefinite article an is used.
If you have studied Spanish for long, you’ll recall that Spanish all nouns have gender (masculine or feminine) and number (singular or plural). Even if you haven’t studied Spanish long, just knowing that nouns have a gender and number and that articles and adjectives have to agree with them in gender and number should enable you to answer this question: If English, a language without gender or number markers has only two articles (the, and a/an), how many definite and how many indefinite articles must Spanish have, being a language with gender and number markers for articles and adjectives?
If you answered four in each case, you are right: the becomes el, los (for masculine singular and plural) and la, las (for feminine singular and plural) and a/an becomes un, unos (for masculine singular and plural), una, unas (for feminine singular and plural).
Going back to our apple examples, una manzana is an apple, and la manzana is the apple. Likewise, the plurals are unas manzanas and las manzanas. Unas here could be interpreted as some or a few.
Be sure to look at the lesson on gender and number agreement.