The three most common types of dependent clauses in Spanish are noun clauses, adjectival clauses, and adverbial clauses.
In this case, the entire clause serves as a noun, usually as the direct object of a verb. It is normally introduced with the conjunction que (that).
Te digo que en Madrid hace mucho frío esta época del año.
I tell you that it is cold in Madrid this time of the year.
The subjunctive mood will be used in the noun clause when the verb that governs the clause is one of influence, emotion, doubt, or negation; the indicative is used when the governing verb is one of truth, certainty, reporting, or affirmation.
Quiero que venga a cenar con nosotros.
I want him to come dine with us.
Here, the entire clause takes on the function of an adjective, usually modifying a noun or pronoun, the antecedent (antecedente). Adjectival clauses are normally introduced by a relative pronoun such as que [which/that] quien (who), el que (which/that/who), or el cual (which/that/who).
Estoy buscando a la secretaria que habla español.
I am looking for the secretary who speaks Spanish.
One uses the subjunctive mood in adjectival clauses when the antecedent is indefinite, nonexistent, or negated; the indicative is used when the antecedent is known or definite.
Estoy buscando una secretaria que hable español.
I am looking for a secretary who speaks Spanish.
This type of clause is usually introduced by an adverbial conjunction; antes de que, cuando, mientras, tan pronto como, etc.
Trabajo mucho cuando me siento bien.
I work a lot when I feel good.
The subjunctive is used for adverbial clauses when the action or state in the clauses is anticipated —i.e., viewed as not yet happening— or hypothetical; the indicative is used when the action or state in the clause is viewed as a customary one or one which has been completed.
Podremos entrar al parque tan pronto como Juan traiga los boletos.
We will be able to enter the park as soon as John brings the tickets.