Breakdown of El prototipo ya estará terminado para cuando llegue el atardecer.
estar
to be (location/state)
llegar
to arrive
terminar
to finish / to end
para
for
Questions & Answers about El prototipo ya estará terminado para cuando llegue el atardecer.
Why do we use the subjunctive llegue here?
Para cuando translates to "by the time." Because the sunset is an anticipated future event that hasn't happened yet, Spanish requires the present subjunctive llegue to express that uncertainty or pending status. If this were a statement about a repeated habit, you would use the indicative.
What does para cuando add that a simple cuando doesn't?
Could I use the future perfect ya habrá terminado instead of ya estará terminado?
Yes, but the nuance changes slightly. Habrá terminado ("will have finished") focuses heavily on the completion of the action itself. Estará terminado ("will be finished") focuses on the resulting state of the prototype at that future deadline. In this sentence, terminado functions as an adjective describing the prototype's condition.
Why is the subject el atardecer placed after the verb llegue?