Spanish Grammar Explained /

Indefinite article

Unlike the definite article (the), the indefinite article (a, an) is used to refer to an unspecified object:
a book, an article
In Spanish, the indefinite articles are un(os) and una(s), as shown in the table below:
un mensaje
unos mensajes
una computadora
unas computadoras
And when do I say uno as in uno mensaje?
Never. Uno is a number, not an article. If you say "uno" + [noun], then you are making a mistake.
'Uno' before a full stop:
- ¿Tienes hermanos?
Do you have siblings?
- Sí, un. → Sí, uno.
Yes, one.
'Un' before a noun:
- ¿Tienes hermanos?
Do you have siblings?
- Si, uno hermano. → Sí, un hermano.
Yes, one/a brother.
Summing up:
• Masculine articles are: el, los, un, unos.
• Feminine articles: la, las, una, unas.
Uno is a number, like in uno, dos, tres, use it before a full stop.