A peculiar little grammar structure of the romance languages says that the parts of the sentence must agree numerically. This includes words like: a, an, the, blue, red, striped, kittens, girls, houses, jeans, etc.
It seems strange at first but it is a fairly easy concept to grasp once a student is in the habit of doing it. Just remember that nouns decide if the describing words need to be plural or singular, and that Spanish is a very descriptive language so even "un," "una," "el," and "la," becoming descriptive.
Most adjectives are turned plural the same way as in English, by adding a "s" or "es" to the end of the word, and the same holds true with making your definite (the) and indefinite (a, an) articles plural, but to make "el" plural you turn it to "los."
|It's a sloth. We'll talk about this guy later.|