How to put Un and Una in a sentence
When not to use Un and Una?
This may help
Here are the most common cases where the article (un, una) shouldn't be used even though it's used in English:
Before an unmodified noun after a form of ser ("to be"), especially in reference to occupation, religion, affiliation or social status (normally, if the noun is modified, the article should be used): Soy profesor. I am a teacher. Él es un buen dentista. He is a good dentist. ¿Eres católica? No, soy una metodista feliz. "Are you a Catholic." "No, I'm a happy Methodist." Es artista. She is an artist. Es una artista que muere de hambre. She is a starving artist.
Before otro ("other"): Quisiera otra taza. I would like another cup. Compró otro coche. He bought another car.
Before mil ("thousand") and cien ("hundred"): Gana mil dólares por mes. He earns a thousand dollars per month. Tiene cien años. She is a hundred years old.
In exclamations using qué ("what"): ¡Qué lástima! What a shame! ¡Qué casa! What a house!
After con ("with") and sin ("without"): Come con cuchara. She eats with a spoon. Escribe sin ordenador. He writes without a computer.
Frequently after forms of tener ("to have"), comprar ("to buy"), llevar ("to wear") and some other verbs when generically referring to things that people would normally have or use one at a time: No tengo coche. I don't have a car. Lleva camisa. He is wearing a shirt. Vamos a comprar casa. We're going to buy a house. ¿Tiene madre? Does he have a mother?
Usually when the noun is unmodified so as to represent something in general.
Necesito teléfono. Since I am not talking about a specific phone (un teléfono celular que puede sacar fotos) "un" is not normally used.
Spanish language omits, in certain cases, the use of the indefinite articles:
When we refer to a persons profession or job: El es profesor (He is a teacher)
When we refer to a persons religion: Ella es budista (She is a Buddhist)
Before thousand (mil) and hundred (cien): Tengo cien dólares ( I have a hundred dollars)
When we use qué in a exclamation: ¡Qué gato! (What a cat!)
Sometimes Spanish seems to avoid using the indefinite articles in many places while English does like in occupations, affiliation, religion, before otro (other), after con (with) and sin (without), usually after tener (have)/ llevar (wear)
Finally, there is one case where we don't use the indefinite article in English where it's needed in Spanish: in a series of two or more words joined by "and" (y in Spanish). In English we might say "a cat and dog," but in Spanish it must be un gato y un perro. Without the second un, the phrase would be understood as referring to one creature, a cross between a cat and dog. Note the distinction in these sentences: Conozco a un artista y un dentista means "I know an artist and I know a dentist," while Conozco a un artista y dentista means "I know a dentist who is also an artist."