Indefinite Articles, Les articles indéfinis
Indefinite articles in French are equivalent to 'a', 'an', or 'some' in English. They are used before nouns to indicate a non-specific person or thing. They also agree in gender and number with the noun.
Rules
Un — used before masculine singular nouns (un garçon = a boy)
Une — used before feminine singular nouns (une fille = a girl)
Des — used before plural nouns (des livres = some books)
In negative sentences, un/une/des become de or d' (Je n'ai pas de chat = I don't have a cat)
Examples
J'ai un frère et une sœur. (I have a brother and a sister.)
Il y a une pomme sur la table. (There is an apple on the table.)
Elle a des amis à Paris. (She has some friends in Paris.)
Je n'ai pas de voiture. (I don't have a car.)
C'est un bon professeur. (He is a good teacher.)
Nous avons des devoirs à faire. (We have some homework to do.)