Translation:As long as you have not learned, we will not go to the beach.
I translated it as "As long as you don't study, we won't go to the beach"
I don't know if it makes sense in English to mix up past and future like in this sentence.
It's normal to combine tenses like that. And, in fact, your sentence makes more sense than duo's. I've sent a comment to them explaining that 'learning' as it is used here requires a direct object, e.g. "....learned your your times tables...etc."
I may be many things other than studying. Maybe the boy has stolen his mother's jewels again, he didn't learn from the last time.
the English here is very awkward. I think we would tend to use an if clause: "If you haven't studied/learnt, we won't go to the beach."
In another sentence I found "zu dem Strand gehen" so I was wondering when I have to say "an den Strand gehen" and when "zu dem Strand gehen"? Or do they mean the same?
They're both correct and nearly synonymous, but ‘an den Strand gehen’ would tend to be used if that's your main destination, whereas ‘zum Strand gehen’ would tend to be used if it's just one stop in a larger itinerary.