As you probably know, the indefinite articles in Spanish (equivalent to ‘a’/‘some’) are un, una, unos, unas. As you probably also know, the definite articles in Spanish (equivalent to ‘the’) are el, la, los, las. The cool thing is they work in the same way in English and Spanish. We use the definite article when we assume the listener knows which concept/thing/place we’re talking about, since a previous reference (explicit or implicit) exists:
A cat is eating pizza.
The cat is eating pizza.
Un gato está comiendo pizza.
El gato está comiendo pizza.
This is a simplification, though, and we use definite articles for other things. If you look up ‘the’ in the Merriam-Webster dictionary, you’ll see there are several uses. Here is an interesting one:
—used as a function word to indicate that a following noun or noun equivalent is a unique or a particular member of its class
We would do the same thing in Spanish:
If you go through all the different uses of ‘the’ in that entry (and there are lots of them) and you translate all the examples into Spanish, all of them will use ‘el/la/los/las.’ But the opposite isn’t true. In Spanish, there’s an extra use: when a common noun (i.e. a noun denoting a concept as opposed to a particular individual) is in the subject position and it has an ‘in general’ implication, it’s always preceded by the definite article:
Cheese [in general] is delicious.
El queso [en general] es delicioso.
Firefighters [in general] work for the city.
Los bomberos [en general] trabajan para la ciudad.
Since common nouns in the subject position always either have an ‘in general’ implication or follow one of the other rules in which an article is always used in both languages, we’ll never ever find in Spanish a sentence beginning with a bare common noun. Remembering this is probably a better way to always get it right, without giving it that much thought.
And, by the way, languages, demonyms, days of the week and seasons are all common nouns in Spanish (and are never capitalized).
Los domingos son muy largos. (Sundays are very long.)
El alemán es muy difícil. (German is very difficult.)
Los alemanes son muy puntuales. (Germans are very punctual.)
El invierno en Chicago es muy duro. (Winter in Chicago is very tough.)
For indefinite articles, there’s also a case in which Spanish and English are different. When we want to say I’m a [profession], we’ll drop the ‘a’ in Spanish, and we’ll just say soy [profesión].
If you remember these two things, you’ll get your articles right 99.9% of the time!
1. Never begin a sentence with a bare common noun. Just throw an el/la/los/las in there!
2. Drop the un/una when you’re trying to say 'someone is a [profession].'