"What is the boy eating?"
Translation:Vad äter pojken?
Vad äter pojken? can in theory mean 'What is eating the boy?'. But with no context, that is a very unlikely interpretation. The idiomatic way of saying that would be Vad är det som äter på pojken? 'What is it that is eating [at] the boy?', which is pretty different.
Vad äter äpplet? sounds odd and if I met that sentence, I would pause and wonder.
Because Swedish swaps the first verb and noun in order to form questions.
"Han kan göra det" - "He can do it"
"Kan han göra det? - "Can he do it?"
"En trevlig man kom i går" - "A nice man came yesterday"
"Kom en trevlig man i går?" - "Did a nice man come yesterday?"
Languages often tend to have different grammar than English. So you shouldn't assume a sentence in Swedish should follow the same rules as English.