cuando llegara al hotel

Question

why is a future tense used in this sentence? could we use some of the past tenses instead?

Answer

Hola Emanuel
In this sentence:

Dijo que me llamaría cuando llegara al hotel.
He said he'd call me when he arrives at the hotel. (future/hypothetical event)

we are expressing something that will happen. He has not yet arrived at the hotel. It is something expected to happen soon. This is why we need the subjunctive here. 
This would be a different situation where you could use the preterite:

Me llamó cuando llegó al hotel.
He called me when he arrived in the hotel. (He did arrive)

Saludos
Inma

Answer

Worth emphasising that "llegara" [no tilde !] is *not* a future construction?