"The girl thinks that the Christmas decorations are really beautiful."
Translation:Pigen synes at julepynten er virkelig smuk.
Maybe not. Since an opinion is being expressed in this sentence, "synes" looks like it should be the appropriate word here. "Tror" on the other hand is used when a sentence expresses a belief. Or at least, that's what Danish people have told me. Though I have to admit I'm not always entirely clear about the distinction between an opinion and a belief.