Whoa Duolingo guys. First off, you've just introduced perfective aspect without actually introducing it. Secondly, some explanation about this všechno term would be really handy. It appears to be a neuter form of the masculine noun všechen; some grammatical explanation of its use in this case (why the neuter, how it is able to function as an adverb, etc.) would be really helpful. I'm pretty amazed there isn't a lengthy discussion about this already.
Is anyone able to help?
Yeah, sorry, I totally didn't realize at the time that the content was created, monitored, and edited by volunteers. I've been much more patient and understanding since having that pointed out!
Thanks for the recommendation on Tahal; the text is available free online, so that's pretty nice :) I've also got a copy of the Routledge Essential Grammar of Czech which I'll start making my way through at some point, perhaps trying to take points in tandem with their presentation in Duolingo.