What is an indefinite article in Spanish?
[] un, una, unos and unas
-> They all mean a, an, one, or some
What determines which one you use?
[] The object or whatever you are describing's "gender" and amount
[] Gender:
-> Un and unos are mascilune.
-> Una and unas are feminine
[] Amount:
-> Use un and una when talking about a single thing
-> Use unos and unas when talking about multiply things
Example:
A book is a single thing. In Spanish book is "libro" and this is masculine.
-> Un libro = a book
Tables are multiple things. In Spanish tables are "mesas" which are feminine.
-> Unas mesas = some tables
Have a nice day!
I hope this is what you are looking for, but if not - comment! I will edit and update my answer accordingly.
- Heather