1. Are used when introducing a noun for the first time (Situaciones p 20-21):
Había una vez un gato Era un señor muy divertido
2. Are often used with an adjective to form a noun:
Un gordo Una francesa
3. Unos and Unas are often used with numbers to indicate an approximation
Hay unos veinte estudiatnes en la clase Saqué unas cien fotos de Madrid
2. The plural articles unos/unas correspond to the English some and a few and are often omitted in Spanish.
Tengo (unos) amigos en Salamanca
3. The indefinite article un/una is omitted:
a. with names of professions after the verbs ser and hacerse unless the profession is modified by an adjective:
Mi prima es abogada Mi prima es una excelente abogada
b. after verbs such as tener, llevar, poseer, and hay when the concept of quantity is not emphasized and especially if the sentence is negative or interrogative:
No tengo coche ¿Por que no llevas sombrero? ¿Hay examen hoy?
c. after con or sin when the concept of quantity is not emphasized:
No salgas sin impermeable
d. with words such as otro, tal, medio, qué
Necesito otro lápiz Dame media taza de café ¡Qué bonita casa!