Subordinating conjunctions introduce a subordinate clause that depends on another main clause. They do not join two elements that are equivalent, as do the Coordinating Conjunctions, but rather indicate that one idea is placed within another. In Quiero que vengas, que vengas cannot stand alone with the same meaning: it needs the main clause Quiero. These conjunctions create relationships of time, cause, condition, purpose, concession, and comparison, and are a central part of the Oraciones Subordinadas.

When, before it, after it and while place an action relative to another. When can refer to a specific moment or to repetition: I'll call you when I arrive; When I was a child, I lived in Seville. Before it marks an action earlier than another: Close the door before you leave. After it indicates a later action: We go out after the rain ended. While presents two actions at the same time: Study while I cook. These conjunctions organize the sequence of facts within the sentence.

Because, since and as introduce the motive or explanation of an idea. The subordinate clause answers the question why. Because usually goes after the cause or direct response: I did not go because I was sick. Since and as present a known or evident cause and are usually placed at the beginning of the explanation: Since you have time, help me; As there are no tickets, we stayed home. If the conjunction is removed, the causal relation disappears and the two ideas are left without a clear connection.

If, unless and provided that express a condition for something to happen or to limit an action. If presents the more general condition: If it rains, we won’t go out. Unless introduces an exception that prevents the result unless something else happens: We won't go unless it finishes soon. Provided that marks an acceptable or sufficient condition: I’ll lend you the car provided that you come back early. In these constructions, the subordinate clause sets the requirement that controls the main clause.

So that and in order that indicate the objective or intention of an action. The subordinate expresses what is done for: I’m writing to you so that you confirm the time; In order that everyone participates, we will send the program in advance. These forms usually appear when a person acts with a clear purpose. The main idea presents the action, and the subordinate explains the goal that that action seeks to achieve.

Although, though and even though introduce a difficulty, a contrast or an obstacle that does not prevent the result. Although can present a real fact or an admitted condition: Although it rains, we go out. Though and even though tend to sound more formal and also announce a strong contrast: Even though he was tired, he kept working; Even though there was a lot of traffic, he arrived on time. The subordinate clause acknowledges the problem, but the main clause shows that the result remains.

Como, tal como and igual que serve to compare actions, forms or ways of doing something. Como can signal similarity in the way: Do it like I told you. Tal como presents an exact match: They explained it as it was in the book. Igual que expresses equivalence: Run as fast as your sister. These conjunctions connect two actions or two ways of acting to show similarity, model or correspondence. In comparative texts, they often bring the subordinate clause closer to the sense of an example or reference.

Many subordinating conjunctions require the subjunctive when the subordinate clause presents an unperformed, desired, doubtful, or future action. Before it, so that, in order that, unless, provided that, even though and although in certain contexts usually require it: I will call you before you leave; I do it so that you understand; I don’t go out unless you call me. With if, the verb changes according to the type of condition: If you come, we go out; If you came, we would go out. When the subordinate clause speaks of a real and known fact, the indicative appears; when it expresses purpose, an uncertain condition or future time relative to the main clause, the subjunctive appears. This relationship between conjunction, certainty, and verb tense is a direct foundation for the Oraciones Subordinadas and for the Modo subjuntivo.