"You want the river to flow rapidly."
Translation:Flumen rapide fluere vis.
I had the same question. In the other forum for this question, there's a good string about the "LATIN INFINITIVE CONSTRUCTION" or something like that. It allowed an infinitive, instead of the subjunctive. I don't know how frequent either construction was; I would have thought subjunctive is much more common since all the Romance languages use subjunctive there.
According to grammar alone, you could translate either sentence either way. "Flumen rapide fluere vis" could mean either "You want the river to flow rapidly" or "You want to flow the river rapidly." "Arbores rapide ascendere vis" could mean either "You want the trees to ascend rapidly" or "You want to ascend rapidly."
But for each of those, we're choosing the translation that makes more sense. You can't flow a river, and you most likely aren't wanting trees to ascend. So these sentences are most naturally interpreted as the river flowing and you climbing the trees.
You don't tend to double up finite verbs in Latin. Often only one is finite (vis) while the other is non-finite, like an infinitive form (fluere). The finite verb already carries most of the needed information for tense, person, number, etc. and the other verb doesn't have to hold that information.