The Subjunctive in Adjectival Clauses

Adjectival or relative clauses function as adjectives modifying a noun or pronoun, which is called the antecedent (antecedente). Adjectival clauses are most frequently introduced by the relative pronouns que (which/that/who) quien/quienes (who, that), el que, la que, los que, los que (which/that/who), or el/la cual, los/las cuales (which/that/who).

The verb in the adjectival clause may be in a tense of the indicative or of the subjunctive, as illustrated in the following examples:

With indicative:
Busco a la secretaria que habla español.
I'm looking for the secretary who speaks Spanish.
Habla is indicative because the antecedent of que is definite: the secretary I am looking for exists (I know she exists).

With subjunctive:
Busco un apartamento que no sea muy caro.
I’m looking for an apartment that is not very expensive.
Sea is subjunctive because the antecedent of que is indefinite or unknown: the apartment I am looking for may actually not exist (that is, I am not talking about an apartment I know).

No tengo padre, ni madre, ni perrito que me ladre.
I don’t have a father, a mother or a dog that may bark at me.
Ladre is subjunctive because the antecedent of que is negated in the main clause: the dog I am talking about does not exist.

(The underlined clauses above function as adjectives because they are modifying their respective antecedents (boldfaced), just like the adjectives española, ladrador and barato in the sentences below:
Busco a la secretaria española.
I'm looking for the Spanish secretary.
Busco un apartamento barato.
I’m looking for a cheap apartment.
No tengo padre, ni madre, ni perrito ladrador.
I don’t have a father, a mother or a barking dog.)

Important detail: In English, the relative pronoun may be omitted in many cases: I lost the book (that) I bought yesterday. In Spanish, this is impossible: Perdí el libro que compré ayer.