"The horse runs quietly."
Translation:말이 조용히 달려요.
Adding 게 gives the meaning of "let" or "make(force)," so it wouldn't really be applied here.
히 rather than 하다 gives a meaning similar to the English -ly
게 also has some other meaning associated to it (its a very complex particle), so here's a link which describes its main usages in detail:
Actually just like "horse" + the subject marker ;) however 마리 is also a word: It's the unit in which you count animals (in Korean all nouns work a bit like English "air". You can't just say "one air", you have to add a unit: "one litre/glassfull/gush... of air". Similarly, you can't say *개 하나 "one dog" in Korean, you have to say 개 한 마리 "one 마리 of dog").