"Tall trees are beautiful"
Translation:Miti mirefu ni mizuri
Just like there are different words for beautiful in english (lovely, pretty, beautiful, glamorous), there are different words in swahili as well. If you found a synonym that you think should be accepted, you should report it to let the course contributors know, they don't check the sentence forums.
I've seen more translations of that to mean pleasing, attractive etc than I've seen of it as beautiful, whereas -zuri comes up as beautiful more often than not.
There is a sentence in the course that uses kupendeza: Wakioga watapendeza. I can't remember what skill it turns up in. The "correct" translation is "If they take shower they will look good". Obviously the English needs fixing, but I put this here to show what sort of context they might use kupendeza in.