No probs! When your answer is judged either correct or incorrect, you should then see a little "message icon" to get to write a comment such as this, as well as a little flag. At least you have both of these for most types of questions, both in the web version and the Android app -- I'm not sure about the iOS app.
Clicking on that flag allows you to report that your answer should have been accepted, that there was something wrong with the sound, that the English or the Finnish is unnatural, etc. The exact choices depend a bit on the type of question. Those messages go to the volunteer contributor team, who can then react to them. They don't necessarily see our messages here -- I think that depends on whether they happen to look.
If you suggest an alternative answer and it is accepted, you get a message (if you have your settings set to allow this). It is very satisfying to receive such reports and know that you personally helped improve the course for future users! (And also that your suggestion was correct, in case you had any doubts.)
I see hiljaa is translated "quietly". I this because this -- the singular partitiivi of adjectives -- is a way to form adverbs, or because hiljaa is agreeing in number and case with lintua? If it is the latter, is there a difference between neljä sinistä lintua laulaa hiljaa and neljä sinistä hiljaa lintua laulaa? That is, what is the grammatical importance of putting the adjective after the verb? This changes the focus of the modifier from the noun to the verb? What are the limits of this? Does this only work with subjects?
Hmm, (not being a linguist) I don't know if there is a general rule for deriving adverbs from adjectives, but at least here this is not the case.
The corresponding adjective is hiljainen, so four quiet birds = neljä hiljaista lintua. The partitive plural is not covered by this course yet.
But yes, quiet birds is a different thing than birds singing quietly.