The boy is lying on a bed is cobsidered wrong... Hmm, even gramatically so? Or more a convention, the notion that the boy is at the end, new info, so therefore he is LIKELY the unknown one? What about "The boy is lying on THE bed"? Will this be accepted... (I am quite confounded at this juncture)...
Technically both your variants are right, it just depends on the context (so you should suggest that these answers are accepted). As others pointed out in the discussion thread, though, there's a subtle emphasis that it's just "a boy" as opposed to "the boy" since it's at the end of the sentence. That's like ultra high level concept though in my opinion, and really it can be translated just as you said.
I put "on the bed is a lying boy." Where did I go wrong on this? I feel that this should be accepted. When I go to learn another language I also try to learn how things are expressed in that language and make my translations fit the target rather than making the target fit my native language. All that said I could still be wrong but I feel that that should be a proper translation?
Languages don't always allow for 100% direct translations, especially when talking about word order in sentences.
"On the bed is a lying boy" doesn't really follow common English word order. Furthermore, the "is" in "is lying" is an example of an auxiliary verb that directly modifies "lying" to show tense. As I read your sentence, I get the sense that the boy on the bed is dishonest (while the sentence is trying to convey that he's actually physically situated on the bed in a specific posture).
In English you do not actually put the important information anywhere, most of the time. It just falls in place wherever grammatically appropriate. English has an extremely rigid word order, so there are few choices involved, regardless of what your point is.
It is like... even in Russian you would typically put an adjective before the noun it modifies (narrow window = узкое окно). This is not a choice you make but rather the standard position you would use 97% of the time. On the other hand, the difference between "У меня в рюкзаке доска" and "Доска у меня в рюкзаке" is usually motivated by what you are actually trying to express (though, just like in English, intonation lets you clarify that even when the word order may obscure the meaning).