I believe all the next four sentences are correct:
A boy is opening a box.
A boy is opening the box.
The boy is opening a box.
The boy is opening the box.
Am I wrong?
Yes, all these are correct. Thanks for reporting it, I fixed the accepted translations.
Значит тут не действует правило, что если "the" то "эту" или "этот" в русском варианте?
It depends on the stress. If the o is not stressed, it takes an ah or uh (schwa) sound. I recommend this site. When I first started studying Russian, I didn't pay a lot of attention to vowel reduction, thinking I'd just pick up on it; turns out I'm not that good. :) As you continue, I recommend sites like ru.Wiktionary.org or openrussian.org, since these sites will indicate which vowel is stressed in a word.
this "not accepting correct answer because of skipped articles" needs to stop. I'm not here to learn english, i'm here to learn russian, and half of my answers are marked wrong because of articles in english. why do i even have to translate so many sentences from russian to english while learning russian? translating from language you are learning to language you already know is 1000 times easier than from language you know to a language you are learning, plus, it's not helping you get proficient in the slightest. I spent so much time trying to learn russian, and still can't form a single sentence on my own, but hell, I can translate it perfectly (minus the articles). the app should focus more on asking me to translate from english to russian, because then my errors would actually mean something and be a point of learning. skipped articles in this day and age, are irrelevant. no one even uses them unless they actually change the meaning.
does открывает sound like "at-kra-baet" or is just my "untrained" ear? or is there another explanation to say "кры" as "kra"?