You put underscores as place holders. "therefore, xx", or "it follows that xx".
I know that double-asterisk means bold, but I guess double underscore does too. (I would thought it means underline...)
This site http://spanish.answers.com/learning-spanish/the-meaning-of-entonces-in-spanish says that one of the uses of "entonces" is as a filler word, basically "um". "Anyway" is used that way in English sometimes.
I think it's meant to follow on from a previous comment, where the reason why it is impossible has been explained, e.g. "If you want to jump across this gap, be my guest, but it's 15 metres wide" - "Then, it's impossible". 'Anyway' would be in contradiction to what was just said, e.g. "Jumping across this gap would make you a hero" - "Anyway, it's impossible" (... not the best examples perhaps)
IF THE WORD THEY USE YOU HAVE TO TRANSLATE, Translate THAT word, don't assume your idea is right and then cuss Duo out for it not being right. If i asked you to translate "They consumed my shortbread!" and you translated it out to where it read "They ate my bread" I'd mark you wrong.
Too bad. It looks like the whole "learning-spanish" section is gone.
This wordreference page is pretty good: http://www.wordreference.com/es/en/translation.asp?spen=entonces