Which definite article (el, la, las, los)? 2

Definite articles with titles and professions — Use the definite article before titles when talking about someone but not when addressing them directly: El doctor García llegó tarde (talking about him) vs Buenos días, doctor García (addressing him). Same with el señor, la señora, el profesor.

Al and del — the two contractions — a + el → al and de + el → del. These contractions are obligatory: Voy al mercado. Vengo del trabajo. They only happen with the article el (no accent), not with the pronoun él: Hablo de él (no contraction). Never write a el or de el.

Definite article with body parts and clothing — Use the definite article — not possessives — with body parts and clothing when the owner is clear: Me duele la cabeza (not mi cabeza). Levanta la mano. Ponte el abrigo. The reflexive or dative pronoun already signals the owner; the possessive would be redundant.