What are indefinite articles?
These are a/an, or un/una.
In spanish, definite articles (el/la) are used much more than in english, while indefinite articles are much less frequently used in spanish.
Profession, Religion, Nationality, Marital Status.: DON'T USE
Es estudiante (He is a student).
Es mexicana (She is a Mexican).
Modifying the noun: USE IT
Es un estudiante muy trabajador. (He is a very hardworking student)
Es un católico religioso. (He is a religious catholic)
Before cierto, cien, mil, otro, medio, semejante, tal, y que ___!" DON'T USE
Hay cien invitados. (There are 100 guests)
Qué lástima!
No tener/No hay: USUALLY NOT
I have no money. (No tengo dinero)
However, if I wanted to say:
I don't have a single dollar. (No tengo un dolar).
After sin: DON'T
Salió sin abrigo.
After con: USUALLY NOT
Escribo con pluma. (I write with a pen)
Escribo con una pluma. (I write with one pen.)