Which indefinite article (un, una, unas, unos)? 1

Un and una agree with the noun — Un precedes masculine singular nouns; una precedes feminine ones: un libro, un hombre, un problema — una mesa, una mujer, una ciudad. The gender of the noun is fixed (it's not a choice) — learn each noun with its article to get the gender right from the start.

Unos and unas — some, a few — Unos (masculine) and unas (feminine) mean "some" or "a few". They are less common than in English — Spanish often omits them where English uses "some": Tengo amigos allí rather than Tengo unos amigos allí. Use unos/unas when you want to stress the small quantity.

Una before stressed a- is still una — Unlike the definite article (which becomes el before feminine nouns starting with stressed a-), the indefinite article stays una: una área, una arma, una aula. Only el → el agua; the indefinite article doesn't follow this rule.