"Who are the men in front of the tall building?"
Translation:Kik a férfiak a magas épület előtt?
After several different comments plus the hints and tips, I think it goes like this:
Omit van / vannak when using present tense, third person (singular and plural) and describing the subject using a noun or an adjective.
Omit van / vannok when using present tense, second person formal (őn, önök), basically same as for third person as the verbs are the same.
Use van / vannak when describing when, how, where something/someone is (adverbs) or past or future tenses.
Please add to this if not correct, as I am still working this out myself.