aunque + indicative or subjunctive

You met aunque as a linking word at B1 (aunque, sin embargo, por eso, así que, además). It can be followed by the indicative or the subjunctive, and the mood changes what the sentence tells the listener. English usually marks the difference with although vs even if.

Aunque + indicative: although (a fact I'm telling you)

The speaker presents the information as true and as worth stating.

• Aunque está lloviendo, vamos a salir a correr.Although it's raining, we're going out for a run.
• Aunque estaba agotada, terminó el informe esa noche.Although she was exhausted, she finished the report that night.
• La película es buena, aunque es demasiado larga.The film is good, although it's too long.

Aunque + present subjunctive: even if (a possibility)

The speaker doesn't know whether it is true or will happen, and says it makes no difference.

• Aunque llueva mañana, saldremos a correr.Even if it rains tomorrow, we'll go for a run.
• No pienso disculparme, aunque se enfade.I'm not going to apologise, even if she gets angry.

Aunque + subjunctive about a known fact: conceding a point

The subjunctive is also used for something both speakers already know, when the speaker treats it as beside the point: "OK, that's true, but…".

• Aunque sea tu hermano, no tienes que darle siempre la razón.Even though he's your brother, you don't always have to agree with him.

Here aunque es tu hermano would also be grammatical; it would sound like you are informing the listener that he is their brother.

Aunque + imperfect or pluperfect subjunctive: even if (unlikely or contrary to fact)

Like si (Hypotheses: si tuviera, iría), the imperfect subjunctive makes the situation imaginary, and the result goes into the conditional.

• Aunque me pagaran el doble, no volvería a esa empresa.Even if they paid me double, I wouldn't go back to that company.
• Aunque hubiera salido antes, no habría llegado a tiempo.Even if I'd left earlier, I wouldn't have arrived on time.

Latin America

The same contrast is used everywhere. You'll also hear aun cuando in formal styles, with the same moods.

Common mistakes

• Aunque llueve mañana, saldremos: a future possibility needs the subjunctive: Aunque llueva mañana.
• Aunque me pagaran el doble, no volveré: an imaginary condition needs a conditional result: no volvería.
• Thinking the subjunctive always means "maybe it isn't true": aunque sea tu hermano concedes a fact.
• Confusing aunque (although, even if) with aun (even) and aún (still, yet): aún no ha llegado = he still hasn't arrived.