Subordinating conjunctions introduce a dependent clause within another main clause. The subordinate clause can express content, condition, cause, time, concession, purpose, or simultaneity, and its meaning is always interpreted in relation to the main clause. These conjunctions connect ideas more precisely than coordinating ones and are common in constructions linked to Conjunctions.

That introduces content-dependent clauses and typically appears after verbs such as think, believe, say, or know, while 'if' introduces an indirect question or a condition stated dependently. In colloquial speech, 'that' is sometimes omitted after think or say, but the subordinating function remains the same. The choice between the indicative and the subjunctive depends on certainty, negation, or projection into the future—a contrast that also helps to understand Causal Conjunctions and other logical relations.

IdeaIdea	EjemploExample
Que introduce contenido dependiente.That introduces dependent content.	Creo que vendrá.I think he will come.
Si introduce una pregunta indirecta.If introduces an indirect question.	Dime si quieres venir.Tell me if you want to come.
En registro coloquial puede omitirse que.In colloquial register, 'that' can be omitted.	Creo vendrá.I think he'll come.

Because and since express cause, but because typically presents a known cause or direct answer, while since provides a more discursive explanation. When it introduces a temporal relation, it places the subordinate action before or simultaneously with the main one. As and puesto que also express cause; as can go at the start of the sentence and sounds more formal, so it is advisable to reserve it for careful registers.

IdeaIdea	EjemploExample
Porque expresa la causa directa.Because expresses the direct cause.	No fui porque estaba enfermo.I didn’t go because I was sick.
Cuando sitúa una acción en el tiempo.When it places an action in time.	Cuando llegues, empieza la reunión.When you arrive, start the meeting.
Como puede abrir una causa con tono formal.As can introduce a cause with a formal tone.	Como no había, me fui.As there wasn’t any, I left.
Ya que presenta una explicación más desarrollada.Since it presents a more developed explanation.	Ya que estabas aquí, te lo conté.Since you were here, I told you.

Although it introduces a concession, that is, a circumstance that does not prevent the main fact; if the situation is hypothetical or future, the subjunctive is usually required. For that, it expresses purpose and requires the subjunctive because the subordinate action has not yet taken place. These relations are especially useful together with Adversative Conjunctions and Consecutive Conjunctions, because they refine the contrast between obstacle, result and purpose.

IdeaIdea	EjemploExample
Aunque admite un obstáculo no decisivo.Even if it is late.	Aunque sea tarde, vendré.Even if it's late, I'll come.
Para que expresa propósito.To express purpose.	Te llamo para que vengas.I'm calling you so that you come.
El subjuntivo aparece cuando la concesión es hipotética.The subjunctive appears when the concession is hypothetical.	Aunque llueva, saldremos.Even if it rains, we'll go out.
La subordinada de finalidad suele ir después de la principal.The subordinate clause of purpose usually goes after the main one.	Estudio para que me entiendan.I study so that they understand me.

While indicates simultaneity and normally presents a simultaneous scene between the main clause and the subordinate clause. 'While' can also reinforce contrast between two parallel facts or between two compared situations. In both cases, the subordinate usually goes after the main verb, although it can also precede to highlight the temporal frame or the opposition.

IdeaIdea	EjemploExample
Mientras indica simultaneidad.While indicates simultaneity.	Mientras cocino, escucho música.While I cook, I listen to music.
Mientras que puede marcar contraste.Whereas it can mark contrast.	Ella trabaja mientras que él descansa.She works while he rests.
La subordinada puede ir antes para dar marco.The subordinate clause can precede to set the frame.	Mientras cocino, escucho música.While I cook, I listen to music.

The subordinate clause is usually placed after the main sentence, but it can be placed before when you want to highlight the cause, time or condition. The position does not change the syntactic relation, although it can modify the informational focus and the tone of the utterance. The choice between indicative and subjunctive depends on whether the information is presented as certain, negated, future or hypothetical, and its concrete use may vary by region and register.

IdeaIdea	EjemploExample
La subordinada suele ir después de la principal.The subordinate clause usually goes after the main one.	Te aviso cuando llegues.I'll warn you when you arrive.
La subordinada puede ir antes para destacar la circunstancia.The subordinate clause can go before to emphasize the circumstance.	Cuando llegues, te aviso.When you arrive, I'll warn you.
El modo depende de certeza o hipótesis.The mood depends on certainty or hypothesis.	Aunque es tarde, sigo trabajando.Even if it's late, I'm still working.
El uso del subjuntivo varía según región y registro.The use of the subjunctive varies by region and register.	Busco a alguien que sepa francés.I'm looking for someone who knows French.

Subordinating conjunctions organize the logical relationship between a main clause and a dependent clause. That and if introduce dependent content or options, because, since, as and given that explain cause; when they place the action in time; although they admit concession; to express purpose; and while they establish simultaneity. The combination of meaning, position, and verbal mood allows constructing precise and natural sentences in Spanish, in continuity with what was studied in Coordinating Conjunctions and Correlative Conjunctions.