The indefinite article a/an (un, una) has some characteristics which may catch Spanish speakers out.
1. When the article is followed by a vowel or silent consonant, the form an must be used:
an interesting book
an honest man
2. Sometimes, the article may accompany nouns that seem to begin with a vowel . The sound, however, is not like that of a vowel, but the semi-vowel /j/. In this case, only the form a can be used:
a university / ə ˌjuːnɪˈvɜːsəti/
a union /ə ˈjuːnjən/
3. In Spanish phrases such as “Tengo casa/perro/gato/coche/etc.” and “Soy profesor/camarero/policía/etc.”, the indefinite article is not used. In English, the article must always be present.
I have a house = Tengo casa
He has a car = Tiene coche
I haven´t got a cat = No tengo gato
I´m a teacher = Soy profesor
She´s a waitress = Es camarera
My uncle is a police officer = Mi tío es policía