Adjectives are words that modify a noun, describing or limiting it. Examples of adjectives are: the new hat, the pretty dress, many intelligent students. An entire clause may serve an adjectival purpose, describing a noun or pronoun —the antecedent— in a sentence, for example:

• Do you have a dress which will go with these shoes?
• Yes, I have a dress which will be perfect.
• I don't see any dress (that) I like.

Note that in all three examples above, there is an adjectival clause that modifies or refers back to the word dress. That is, the antecedent in all three cases is dress, but in each case the situation is different: in number one, the antecedent is indefinite, that is, we don't know if such a dress exists; in number two, the antecedent is definite, that is, it definitely is portrayed as existing; in number three, the antecedent is negated, that is, its existence is denied or at least doubtful.

The rule: In Spanish, the subjunctive is used in an adjectival clause when the antecedent is indefinite or unknown or is nonexistent or negated; in contrast, the indicative is used when the antecedent is a definite or existing one.

• Tengo un perro que ladra mucho.
  Indicative - There is a definite antecedent, a dog which I own dog.
• No tengo un perro que ladre mucho.
  Subjunctive - The antecedent is negated; such a dog doesn't exist.
• Quiero un perro que ladre mucho.
  Subjunctive - There is an indefinite antecedent; such a dog may or not exist.
  (I have/don't have/want a dog that barks a lot.)

More examples of the three types of situations:

The indicative is used in an adjectival clause when there is a definite antecedent:

• Hay algo aquí que me gusta.
  There is something here which I like.
• ¿Conoces al profesor que vive allí?
  Do you know the professor who lives there?
• Leo un libro que explica todo eso.
  I'm reading a book which explains all that.
• Tienen una criada que habla español.
  They have a maid who speaks Spanish.

The subjunctive is used in an adjectival clause when antecedent is negated:

• No hay nada aquí que me guste.
  There is nothing here I like.
• No veo *a nadie que conozca.
  I don't see anyone I know.
• No recomendamos ningún libro que él haya escrito.
  We don't recommend any book he has written.

The subjunctive is used in an adjectival clause when there is an indefinite antecedent:

• ¿Hay algo aquí que te guste?
  Is there anything here you like?
• Quiero leer un libro que explique todo eso.
  I want to read a book which explains all that.
• Conoces *a alguien que viva cerca de aquí?
  Do you know anyone who lives nearby?
• Buscamos una criada que hable español.
  We're looking a maid who speak Spanish. [We hope we can find one!]

*Note the use of the personal a before the pronouns alguien and nadie when used as direct objects.