Unlike English, every noun in Spanish has a gender. You can't just say 'the cat'; you have to know if it's a boy cat (el gato) or a girl cat (la gata). This is one of the most fundamental rules you must learn to speak correctly.
In Spanish, nouns are either masculine or feminine, and either singular or plural. The 'article' (the word for 'the') must always match the noun's gender and number.
The biggest pitfall for many learners is forgetting that objects have genders too! 'The table' is 'la mesa' (feminine), and 'the book' is 'el libro' (masculine). A good trick: words ending in '-o' are usually masculine, and words ending in '-a' are usually feminine.
Select the correct article to match the noun's gender and number.
Test your knowledge of nouns and articles!
Fun fact: There are exceptions to the '-o/-a' rule! For example, 'el mapa' (the map) is masculine despite ending in '-a' because it comes from Greek, and 'la mano' (the hand) is feminine despite ending in '-o' from its Latin roots.