"The cat is eating a red apple."
Translation:Le chat mange une pomme rouge.
In French, every noun has an assigned grammatical "gender" (either masculine or feminine), and the articles and adjectives used with a noun depend on its "gender".
here, "chat" is a masculine noun while "pomme" is a feminine noun, and the articles used are different:
the cat = le chat / a cat = un chat
the apple = la pomme / an apple = une pomme
There are also special articles for nouns in the plural:
the cats = les chats / (a) cats = des chats