The difference between an "event still to happen" and one that "will happen" can be determined only by the ability to predict the future. It is not a grammatical issue. So neither the indicative nor subjunctive choice is grammatically incorrect. It's a semantic difference, not a grammatical one. I've seen you make very different choices -- different from what I thought was meant.
I'm not sure that you understood correctly when reading the lesson.
The difference between using the indicative and using the subjunctive with cuando is 1. talking about a habitual event, i.e. something that actually happens, as in "everytime I....", and 2. talking about a future event.
You can also see by reading what the main clause says if that is something that is a habitual event or something still to happen. For example, using the future tense or the imperative in the main clause indicates a still to happen action in the cuando clause:
Visitaré a mi abuela cuando vaya a Málaga.
Visita el Museo del Prado cuando vayas a Madrid.
All these are still to happen.
Yo visito a mi abuela cuando voy a Málaga.
This is a habitual event (I visit my grandma every time I go to Málaga)
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