Cuando sometimes takes the indicative and sometimes the subjunctive, and my textbook lists both without explaining which situation calls for which. I end up guessing based on whether the sentence feels hypothetical.
Is there a reliable rule for choosing?
Accepted Answer
The test is whether the event has happened or habitually happens, against whether it is still ahead and unrealised.
Realised or habitual takes the indicative. Cuando llego a casa, ceno. Cuando llegué, ya se habían ido.
Not yet happened takes the subjunctive. Cuando llegues, llámame. The arrival is anticipated rather than actual, so it is subjunctive even though you fully expect it to happen.
That last point is the one that catches people. It is not about doubt. You can be completely certain and still use the subjunctive, because the trigger is that the event is unrealised rather than uncertain.
The same logic runs through hasta que, en cuanto, tan pronto como and antes de que, which always takes it.
Worth correcting the shortcut that subjunctive means doubt or emotion, because cuando is exactly where it breaks.
Mañana, cuando salga el sol, saldremos. Nobody doubts the sun will rise. It is subjunctive because it has not happened yet.
If you carry the doubt rule forward from the verbs of wishing and feeling, every future time clause will look wrong to you and you will keep second guessing correct sentences.
A quick check in practice: if the main clause is future or an imperative, cuando will almost always want the subjunctive. It is not universal but it catches most of the cases you will meet speaking.
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