Translation:Ezeket az épületeket?
The difference between "ezek az épületek" and "ezeket az épületeket" is that the second would be used as a response to a question like "Which buildings?" and the first is just a phrase with no context, right?
No, it's not that simple. The first is in the nominative case, the second in the accusative. The first can appear as a subject of a sentence, as in "ezek as épületek pirosak"/"these buildings are red". The second cannot appear as a subject but can appear as an object, as in "ezeket az épületeket látom"/"I see these buildings".
Both in English and Hungarian, both of these can be an appropriate response to "which buildings", as in "which buildings [are red]?" and "which buildings [do you see]?".
Without context, both "ezek az épületek" and "ezeket az épületeket" are correct translations of "these buildings". If this sentence occurs in the Accusative skill, you should probably use the accusative, however ;-)
Because of Hungarian grammar :)
Also, Ezek épületek. would mean "These are buildings." rather than "these buildings".