Cuando indicative vs. subjunctive for future event

Question

For the question: ________ del colegio tendrás que hacer tus deberes.When you get back from school you will have to do your homework.Cuando volverás*Cuando hayas vuelto*Cuando volveríasCuando volvisteI still don't understand why "volverás" is wrong. In another Kwiziq lesson under:  Learn about como, cuando, donde and quien in Spanish, one of the examples is as follows:Hablo con ella cuando llega a
casa.I speak with her when she gets home. [specific situation] Hablaré con ella cuando llegue a casa.

I will speak to her whenever she arrives home. [We don't
know when she will arrive]In the first case, the indicative is used even for a future event (ie. the event isn't until she comes home).  Why is future then note used for "cuando hayas vuelto"?Thank you!

Silvia Piriz · Accepted Answer

Hola M R.
Thanks for your thoughtful question! The difference between using the indicative and the subjunctive with "cuando" can definitely be confusing, especially when referring to future actions.
In the sentence “Cuando ______ del colegio tendrás que hacer tus deberes” ("When you get back from school, you will have to do your homework"), we are talking about two future actions.
First, the person will return from school, and only then will they have to do their homework. Since the return must be completed before the next future action takes place, Spanish uses the present perfect subjunctive: "cuando hayas vuelto". This tense emphasizes that the first action must be finished before the second one can begin.
Now, why not use "volverás" (future indicative)? While English might say “when you will return” or “when you return”, Spanish doesn’t use the future tense directly after "cuando" in time clauses. Instead, when the action is anticipated or has not yet occurred, Spanish uses the subjunctive. If we were describing a habitual or past action, the indicative would be used. But in this case, we are referring to an uncertain or not-yet-completed event in the future.
This links directly to the examples you mentioned from the other lesson. In "Hablo con ella cuando llega a casa", the present indicative "llega" is correct because it describes a habitual or regularly occurring event. In contrast, "Hablaré con ella cuando llegue a casa" uses the present subjunctive "llegue", because the arrival is an anticipated, future event, not something that’s already happening or guaranteed.
To sum up, "cuando hayas vuelto" fits because it refers to a completed action in the future that precedes another. Using the future indicative "volverás" would be grammatically incorrect in this context, even if it seems logical from an English perspective. Spanish handles these future time clauses differently.
I hope you find this explanation useful!
Hasta pronto 
Silvia