Translation:The pain will have gone away after I take the medicine.
Sorry. I can't think of any situation when this construction would be used in English. It is extremely clumsy. And nearly meaningless. Maybe "The pain will go away after I take the medicine." Or, "The pain will have gone away by the time I take the medicine." Or even, "The pain will pass because I take (or took) the medicine" But the way Duo wrote it, I can't see it. I'm open to suggestions, though. I love nothing more than to be proven wrong.
Agreed. I'm no expert but I can't imagine this is any more a reasonable thing to say in Portuguese than it is in English.
A dor terá passado antes de eu tomar o remédio
would make sense.
It's the personal infinitive. You can also say "depois de tomar o remédio." = after taking the medicine
I tried to report this, but it won't let me submit since I started using the new design, so I'll post it here: should or should they not accept "the pain will have subsided..."?
Nope, "passado" in this context mean gone, subsided would be accepted if: "a dor teria diminuído..." or "a dor teria reduzido...".
The word "away" is not necessary in this sentence. It is implied. Yet it is marked as a mistake when you don't put it in. I have sent feedback about this several times; no change.