"Why did you waste the time?"
Translation:למה בזבזת את הזמן?
Should we be teaching מדוע here? I used that word for the last time in the market once and got laughed at for it because it was so ancient.
It's rare in daily speech, but is still common in writing (newspapers, books, etc.) and in formal speech, so I think it should definitely be taught. Just with a warning. :-)
Not really wrong, but in the past tense the pronoun isn't used. You would use אתה if you were emphasising "Why did YOU waste the time?"