"already" means something occurred sooner than you had expected it to.
"yet" means "until now", normally with an expectation that something will happen.
So you can say "She already ate the candy", but not "She yet ate the candy."
And you can say "Has she eaten the candy yet?" and it will mean you're simply asking if it has happened - but you can also say "Has she eaten the candy already?" and it will mean you're expressing surprise that she ate the candy sooner than expected.
Swedish redan works like "already", and än like "yet". They don't correspond 100% perfectly, but they generally work the same way.