In Spanish, countable and concrete nouns are always used with an article.
The indefinite article un, una indicates one of the possible elements, while the definite article indicates the only possible element. Compare:
Quiero vender el coche = I want to sell my car (the only car I have)
Quiero vender un coche = I want to sell one of the cars I posess
Which sentence is correct, el hotel de Madrid or un hotel de Madrid ? As there are many hotels in Madrid, we must use the second option. In the same way, in
Este es un coche caro = This is an expensive car
we use indefinite article because there are many expensive cars in the world, and this car is one of them.
The indefinite article can be used with proper names when they indicate a category of objects:
Tengo un Renault = I have a Renault
Unlike in English, Spanish article has plural forms unos, unas, which indicate some elements of a set. They can be translated as some:
Mañana van a venir unas amigas de la facultad =
Tomorrow some college friends are coming
Like in English, the indefinite article must not be used with abstract or not countable nouns after the principal verb:
Tengo ___ confianza = I have ___ faith
Fumo ___ tabaco = I smoke ___ tobacco
However, if we add an adjective, the article appears:
Tengo una confianza absoluta = I have an absolute faith
Fumo un tabaco malísimo = I smoke a terrible tobacco
Unlike in English, the indefinite article appears with rations that we eat or drink, compare:
Normalmente no tomo ___ té = Usually I don`t drink ___ tea
Voy a tomarme un té = Voy a tomarme una taza de té = I'm going to have ___ tea