Definite articles in noun phrases (el de, la de, etc.) 1

El de / la de — the one from/of — Spanish uses the definite article + de where English uses a possessive noun phrase or "the one with/from": el de Juan (Juan's / the one belonging to Juan), la de Madrid (the one from Madrid). The article agrees with the noun it replaces, not with the person.

Al and del — the two mandatory contractions — a + el → al and de + el → del are mandatory in Spanish — they are never written as two words. Voy al mercado. Es el coche del vecino. The contraction only applies to the article el (no accent), never to the pronoun él (with accent): el libro de él does not contract.

Article as pronoun: los de arriba — The definite article can replace an entire noun phrase: Los de arriba hacen ruido (The ones upstairs are making noise). Las de ayer eran mejores (Yesterday's were better). This is extremely common in spoken Spanish and much more economical than repeating the noun.