Noun gender
In English a book is just a book. In Spanish every noun belongs to one of exactly two genders, masculine or feminine, and that gender decides the article standing in front of it — el or la — and the ending of any adjective attached to it. There is no neuter and no third option to worry about. Best of all, the last letter of the word usually tells you which one you are holding.
The -o / -a shortcut
A noun ending in -o is masculine almost every time: el libro, el cuarto, el gato, el carro, el jugo. A noun ending in -a is feminine almost every time: la casa, la mesa, la papa, la tienda, la computadora. Those two endings cover most of the nouns you will meet in your first month, so start there and trust them.
Endings that tell you nothing
Nouns ending in -e or in a consonant give no signal at all, so you learn the gender together with the word. Masculine: el celular, el café, el hombre, el pan. Feminine: la noche, la calle, la ciudad, la mujer. The habit that saves you months: never memorize mesa on its own, memorize la mesa. Store the little article as part of the word and you will never have to guess later.
The ones that lie to you
A short list of nouns wears the wrong ending. el día, el mapa, el problema, el idioma, el clima all finish in -a and are all masculine — most of them arrived in Spanish from Greek. Going the other way, la mano finishes in -o and is feminine, and the same goes for la foto and la moto, which are just short for la fotografía and la motocicleta. At this level that is nearly the whole list.
Adjectives copy the noun
An adjective takes the gender of the noun it describes. nuevo becomes nueva, viejo becomes vieja, pequeño becomes pequeña: you say el libro nuevo but la mesa nueva, el cuarto pequeño but la casa pequeña. Adjectives ending in -e or a consonant have a single form for both genders — grande and amable never change: el perro grande, la mesa grande. For the plural you add -s: libros nuevos, casas pequeñas.
People, animals, and the -ito habit
For people and for the animals we keep around, there are two separate words: el chico and la chica, el amigo and la amiga, el perro and la perra, el señor and la señora. When a group is mixed, Spanish reaches for the masculine plural — los amigos can easily be four women and one man. The ending -ito / -ita, which you will hear constantly all over Latin America, agrees exactly the same way: el perrito, la mesita, un cafecito, ahorita. It only sometimes means small; most of the time it means warm, friendly or politely softened.
