It could be just where I'm from or some aspect of the sentence I missed but, as a native speaker, I would just as easily, if not more frequently, say "Do this exercise for next Wednesday" and mean the exact same thing as "...by next Wednesday". I didn't report it because I suspect I'm at fault here (Duolingo didn't accept my version) but if someone could help clarify, it would be much appreciated!
That has a different meaning - if you say "do this exercise until next Wednesday", it would mean that the person would keep doing the exercise [perhaps without stopping!] until Wednesday came. "Do the exercise by Wednesday" just means that the person will have to complete it at some stage between now and Wednesday.
Well, yes, but it's closer to the English 'to'.
К 'by' is used to set the final timeline, while до 'to' can be combined with с 'from' to get an interval of time.
When I first answered this, I wrote "Finish this exercise until the next wednesday" and it gave "Finish this exercise by the next wednesday", underlining "by". I copy-pasted the exact same sentence given as the correct form, but Duo gave this also as a wrong again, giving me "Do this task by next Wednesday". It's contradicting its own correct sentence.