"The boys write well."
Translation:La knaboj bone skribas.
When you say "Mine said" do you mean "Duolingo's correct answer said"? If so, are you sure it said, "...bone skribus"? I ask because the -us ending makes the verb mean "would write well" (if something else were true, for example, if they were not very tired, if they had better pens). At this stage of the course, this so-called "conditional tense" has not been covered, so I wouldn't expect them to use skribus.