"Hilla is a quiet girl."
Translation:Hilla on hiljainen tyttö.
If you are talking about people, the adjectives vaitelias and vaitonainen usually mean "silent", and sometimes "reticent", or "taciturn". We accept vaitelias and vaitonainen whenever the sentence is about a quiet person. If the sentence is about something else, for example a quiet forest, only "hiljainen" works. If your answer is not accepted despite this, please flag the exercise, so that we can add any missing translations to the system. :)