"They built a long bridge over the wide river."
Translation:Oni konstruis longan ponton super la larĝa rivero.
Ok, I'll report it next time. Can you explain why oni would work? I can't find the duolingo notes for it, but wiktionary says it translates to "one": 'Comparable to the use of generic "you" in English (e.g. In America, you can do what you want). A sentence whose subject is "oni" can often be translated as an English sentence in the passive voice, for example: "Oni ofte referencas al Kimrio kiel la "lando de la kanto" can be translated as "Wales is often referred to as the "land of song".'
I think it makes sense as a translation of someone saying "they built a bridge" without having much of a clear sense of who the "they" is... but I'll be honest, if you asked me to pick the most natural english translation for "Oni konstruis longan ponton super la larĝa rivero" I probably would pick the passive "a long bridge was built over the wide river" sooner than "they built...". On the other hand oni can in some contexts be a spot-on translation for "they", like of the more clearly impersonal/vague "they" in english "you know what they always say", or "they oughta make a law"
Yes, you can say it both ways. But since each entry has to be entered into the database by hand, there's a lot of valid translations that the system won't recognize.
The subject of 'built' should be 'they' and the direct object should be that which is built, ie the bridge (which is how it is in the Esperanto solution).