Definite and indefinite articles
English has 'the' and 'a'. Spanish has four of each, because the article also carries the noun's gender and number: el, la, los, las point at a specific thing; un, una, unos, unas introduce one of many. Learn every noun with its article stuck to it — la casa, not bare casa — and you get the gender for free.
Definite articles (el, la, los, las)
el = the (masc. sing.): el gato. la = the (fem. sing.): la casa. los = the (masc. plural, and any mixed group): los amigos. las = the (fem. plural): las plantas. Use them when you and your listener can both point at the thing. One quirk: a feminine noun that starts with a stressed a- borrows el in the singular, for sound only — el agua, el área — and stays feminine underneath: el agua fría, las aguas.
Indefinite articles (un, una, unos, unas)
un = a / one (masc.): un carro — a car, the everyday word from Mexico to Chile (un auto is just as pan-regional). una = a / one (fem.): una casa. unos = some (masc. plural): unos amigos. unas = some (fem. plural): unas plantas. Use these when the thing is new to the conversation, or when which one it is simply doesn't matter.
First mention takes un, the second takes el
The first time something shows up you introduce it with un / una. After that you both know which one it is, so it switches to el / la: Tengo un gato. El gato es negro. — 'I have a cat. The cat is black.' Exactly the English move from 'a' to 'the', and it works in the plural too: unas plantas first, las plantas after.
Jobs, titles, and when the article vanishes
After ser + a bare profession there is no article: Lucía es doctora — Lucía is a doctor. Add a description and it comes back: Lucía es una doctora muy amable. Titles work the other way around, and Latin America uses them constantly. Talking ABOUT the person, the title keeps its article: el señor Ramírez, la señora Peña, la doctora Lucía, el ingeniero Torres. Talking TO them, it drops: Buenos días, señor Ramírez. And don / doña + first name — the warm, respectful everyday form — never takes an article: doña Rosa, don Pedro, never la doña Rosa.
