"Those who are running behind the tram are students."
Translation:Azok diákok, akik a villamos mögött futnak.
"Those students who are running behind the tram." seems more natural in English, but I don't think it is really a complete sentence in English. You could say "Those students, who are running behind the tram, are fast." "Who are running behind the tram" serves as an adjectival clause that modifies "students" (I had to look it up to make sure I gave the correct term.) so the noun "students" still needs a verb. The way they have translated the English, "who are running behind the tram" is the adjectival clause modifying "those" so the sentence is complete because the noun has the verb "are".
The English here is not natural, but I think that this is a good example of one of the biggest challenges in translating languages.Sometimes it can be very difficult to express a thought in a way that is natural in both languages Hungarian doesn't seem to have the same requirements for what constitutes a complete sentence as English does.