A sentence expresses a complete thought. Consider the simple sentence below:

Rhonda studied in the Starbucks for her math quiz.

A dependent clause is a group of words that contains a subject and verb but does not express a complete thought. A dependent clause is not a sentence. Read the dependent clause below:

...when Rhonda studied in the Starbucks for her math quiz ...

What happened when Rhonda studied? The thought is incomplete.

Often a dependent clause is marked by a dependent marker word. In the dependent clause above, "when" is the dependent marker word. A dependent marker word is a word added to the beginning of an independent clause that makes it into a dependent clause.

In Spanish, dependent clauses are introduced by que or another subordinating conjunction. The three most common types of dependent clauses are noun clauses, adjectival clauses, and adverbial clauses.

Noun Clauses

A noun clause is one which functions as a noun, normally as the direct object or the subject of a verb. It is usually introduced by the conjunction que (that).

Te digo que en Madrid hace mucho frío.
I tell you that it is very cold in Madrid.

Adjectival Clauses

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

Busco a la secretaria que habla español.
I'm looking for the secretary who speaks Spanish.

Adverbial Clauses

Adverbial clauses are introduced by conjunctions, such as para que (so that), antes de que (before), and hasta que (until).

Ahorra su dinero para poder comprar un coche.
He's saving his money so he can buy a car,