"The third tree is greener than the fourth and the fifth."
Translation:La tria arbo estas pli verda ol la kvara kaj la kvina.
I typed 'La tria arbo estas pli verda ol la kvara kaj kvina' (forgot the 'la' before kvina) and it is marked wrong. A bit harsh imho :)
They must have taken your comment to heart, because I just made the same mistake but it was marked as correct with "...kaj la kvina" shown as an alternate correct solution.
Pli is "more", plej is "most". PMEG has a very thorough explanation about them (it's fully in Esperanto, but you'll understand how to use them even if you don't understand every word): http://bertilow.com/pmeg/gramatiko/e-vortecaj_vortetoj/ceteraj/pli_plej.html