Here's why:
* Grammar: French nouns have grammatical genders, meaning they are either masculine or feminine. This is often unrelated to the actual gender of the thing they represent.
* Agreement: Adjectives and articles (like "le", "la", "un", "une") change form to agree in gender with the noun they modify. Since "an" is masculine, you would use a masculine adjective or article with it.
For example:
* "Un an" (one year) - "Un" is masculine, agreeing with "an".
* "Le nouvel an" (the new year) - "Le" is masculine, agreeing with "an".