"Who are you buying this horse for?"
Translation:Pro koho kupujete tohohle koně?
Kupovat is imperfective and koupit is perfective.
With perfective verbs the morfologically present form is actually the future tense, so "Pro koho koupíš tohohle koně?" is "Who will you buy this horse for?" (also note the simple tense instead of the continuous one for the perfective verb)
I wrote: "Pro koho tohohle koně kupujete?"
Got told it was wrong.
Correct solution: "Pro koho kupujete tohohle koně?"
As far as I understand both should be correct.