Which definite article (el, la)? 1

El and la — choosing the right gender — El is the masculine definite article; la is feminine. Most nouns ending in -o are masculine (take el); most ending in -a are feminine (take la). But there are important exceptions: el día, el mapa, el problema are masculine despite ending in -a; la mano is feminine despite ending in -o.

El with feminine nouns starting with á- — To avoid two vowel sounds clashing, la changes to el before singular feminine nouns starting with stressed a- or ha-: el agua, el arma, el hambre, el alma. The plural reverts to las: las aguas. And adjectives still agree in the feminine: el agua fría.

Always learn nouns with their article — Since Spanish gender doesn't follow a single rule, the safest habit is to learn every new noun with its article: el libro, not just libro. This automatically encodes the gender in your memory. Dictionaries always list the article alongside the noun for exactly this reason.