Definite articles are used when we talk about something specific. There are four definite articles: La (femenine, singular), Las (femenine, plural), El (masculine, singular), Los (masculine, plural). For example: el coche de mi padre es rojo  (My dad's car is red) . As we are referring to a specific car we use the definite article in singular and masculine because car is masculine.

Indfinite articles are used when we talk about something that is not specific. There are four indefinite articles: una (femenine, singular), unas (femenine, plural), un (masculine, singular), unos (masculine, plural). For example: Ayer fui a un restaurante (yesterday I went to a restaurant). We use the indefinite article because we do not talk about a specific restaurant and we use un because restaurant is masculine and singular.