Here's how they work:
* "Un" is used before masculine singular nouns:
* Un libro (a book)
* Un perro (a dog)
* Un coche (a car)
* "Una" is used before feminine singular nouns:
* Una mesa (a table)
* Una casa (a house)
* Una flor (a flower)
Important Note:
* "Un" and "una" are only used before singular nouns. For plural nouns, you don't use any articles.
* Los libros (the books)
* Las casas (the houses)
Let me know if you have any other questions!