el/la, un/una, and how gender works.

• elDefinite article for masculine singular nouns ("the").el libro = the book
• laDefinite article for feminine singular nouns ("the").la mesa = the table
• unIndefinite article for masculine singular nouns ("a/an").un perro = a dog
• unaIndefinite article for feminine singular nouns ("a/an").una casa = a house
• Nouns ending in -oUsually masculine; pair them with el/un.el libro, un vaso = the book, a glass
• Nouns ending in -aUsually feminine; pair them with la/una.la mesa, una silla = the table, a chair
• losDefinite article for masculine plural nouns ("the").los libros = the books
• lasDefinite article for feminine plural nouns ("the").las mesas = the tables
• unos / unasIndefinite plural articles meaning "some" (unos = masc, unas = fem).unos perros, unas casas = some dogs, some houses
• Plural: noun ending in a vowel → + sAdd -s when the singular noun ends in a vowel.libro → libros = book → books
• Plural: noun ending in a consonant → + esAdd -es when the singular noun ends in a consonant.ciudad → ciudades = city → cities
• de + el → delMandatory contraction of the preposition de and the article el.la puerta del coche = the door of the car
• a + el → alMandatory contraction of the preposition a and the article el.voy al cine = I'm going to the cinema
• el díaException: masculine despite the -a ending.el día = the day
• la manoException: feminine despite the -o ending.la mano = the hand
• el problema / el mapaExceptions: masculine despite the -a ending (many Greek-origin words in -ma).el problema, el mapa = the problem, the map