In short

Spanish has definite articles el, la, los, las (“the”) and indefinite ones un, una, unos, unas (“a / some”). They agree with the noun in gender and number, and a + el and de + el always contract to al and del.

The rule

Choosing an article is really choosing the gender you learned in Lesson 1, then matching number. Definite articles point to something specific or known; indefinite ones introduce something new (with unos/unas = “some, a few”). Before a number, unos/unas means “about, roughly” (unos veinte minutos), and unos cuantos / unas cuantas is “a few” (unas cuantas sillas).

Step - Contractions. Two contractions are obligatory in both writing and speech:

• a + el -> al: Voy al cine. (but a la, a los, a las stay separate: a la playa)
• de + el -> del: Vengo del trabajo. (but de la, de los, de las stay: de la casa)

The “el agua” rule. A feminine noun that begins with a stressed a- or ha- takes el / un in the singular, purely for sound: el agua, el águila, el hacha, el aula, el hambre. The noun stays feminine, so:

• adjectives are feminine: el agua fría.
• the plural uses las: las aguas.
• other feminine words come back: mucha agua, esta agua, la misma agua.
• only a stressed a- triggers it: la amiga, la harina keep la.

Omission - where English has an article but Spanish drops it:

• profession, nationality, or religion after ser: Soy médico. Es española. (add one only with an adjective: Es una médica excelente.)
• after otro, medio, cierto, tal, mil: otro café (never “un otro”), medio litro.
• often before generic or uncountable nouns: No tengo dinero. Hay flores.

Addition - where English drops an article but Spanish keeps it:

• a generic or abstract noun as subject: El azúcar cuesta poco. Los perros ladran.
• days of the week, with no en: el jueves = “on Thursday”, los jueves = “on Thursdays”.

The article without its noun. Once the noun is known, el / la / los / las can stand alone - before de, before an adjective, or before demás - the way English says “the one(s)”: Mi móvil es más lento que el de mi hermana. De estas dos cajas, me llevo la pequeña. Las demás siguen aquí.

The neuter article lo. lo is not used with nouns; it turns an adjective or a clause into an abstract idea:

• lo + masculine-singular adjective = “the … thing / part”: lo bueno (the good thing), lo importante, lo difícil.
• lo que = “what / the thing that”: No entiendo lo que dices.

Forms & patterns

The four-way agreement:

The two contractions:

Feminine nouns that take el / un in the singular:

Examples

Common mistakes

• a el cine -> al cine - a + el always contracts to al.
• de el niño -> del niño - de + el always contracts to del. (But de él with an accent, = “of him”, does not contract.)
• la agua / una agua -> el agua, un poco de agua - a feminine noun with a stressed initial a- takes el/un in the singular; it is still feminine (el agua fría, las aguas).
• un otro café -> otro café - never put un/una before otro; “another” is simply otro/otra.
• Soy un médico -> Soy médico - drop the article for an unmodified profession after ser (add one only with an adjective: soy un médico joven).

Tips

• Choosing an article = recalling the noun’s gender (Lesson 1); if you learned la mano, you’ll say la mano, not el mano.
• al and del are the only contractions - there is no “a la” contraction; it stays two words.
• Say el agua but think “feminine”: every agreeing word (fría, esta, mucha) and the plural (las aguas) stay feminine.
• Spanish drops “a/an” where English needs it (Soy médico) and keeps “the” where English drops it (El azúcar cuesta poco, los jueves) - never copy English article for article.
• lo is not “the”: use it for “the … thing” about a quality (lo mejor = the best thing), never before a noun.