Learn Spanish (Latin America): Para cuando evaluemos la maqueta, el prototipo estará terminado. - By the time we evaluate the mock-up, the prototype will be finished.

The phrase para cuando (by the time) introduces a deadline. Because evaluating the mock-up is a future, anticipated event that hasn't happened yet, Spanish requires the subjunctive. If it were a recurring habit, you would use the indicative.

Why do we use the present subjunctive (evaluemos) to talk about a future event?

Spanish doesn't use a future subjunctive in everyday speech. When temporal clauses like para cuando point to a future deadline, you must use the present subjunctive. The future tense in the main clause (estará) is what makes the future timeline clear.

Why does the sentence use estará terminado instead of the future perfect habrá terminado?

Using estar + the past participle (terminado) focuses on the resulting state: the prototype will be in a completed condition. If you used habrá terminado, it would mean the prototype itself actively finished an action. To literally say "will have been finished," you'd need the passive voice (habrá sido terminado), but estará terminado is much more natural in Spanish.

Are maqueta and prototipo interchangeable here?