Using un, una to say a/an (singular indefinite articles)

Look at these examples.

Ana compra una planta.Ana buys a plant.

Eso es un paraguas.That is an umbrella.

Es un excelente actor.He is an excellent actor.

¡Tengo una idea!I have an idea!

Note that to express a/an + [singular noun] in Spanish, you use either un or una, depending on the gender of the noun it refers to, as follows:

una for feminine singular nouns
un for masculine singular nouns

You can never use uno, instead of un.

Tengo uno coche.

Tengo un coche.

Uno is not an article, but the actual number one, and it works as a pronoun. See also Using numbers as pronouns.
It is important to remember that in Spanish, like in all Latin languages, every noun is either masculine or feminine.
and Using los or las to say the (plural definite articles)

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Examples and resources

Ana compra una planta.Ana buys a plant.
Es un excelente actor.He is an excellent actor.
¡Tengo una idea!I have an idea!
Eso es un paraguas.That is an umbrella.
Let me take a look at that...