I answered: "Those trees are beautiful" and it was marked incorrect. Is "beautiful" really so wrong instead "pretty"?
Yes. Hezký does NOT mean 'beautiful', it's not that strong word. Use krásný for 'beautiful'.
I answered 'The trees are pretty' and was marked wrong. I thought 'the' and 'those' were more or less interchangeably translated into English from Czech?
"The trees are pretty" is accepted." Since you didn't also Report the sentence, we can't tell you why what you wrote was rejected. If you're fairly sure your answer is right, please use the Report button, even if you also comment.
"The trees are handsome" should be accepted, too. It's an older usage, but "handsome" can be applied to objects to mean "well-formed and nice-looking".