"Is a boy eating the lemon?"
Translation:Spiser en dreng citronen?
Questions work the same way in all Germanic languages including English and Danish. Normally you have to invert the word order, i.e. put the verb first. Only English has the complication that for full verb we often use do periphrasis and it is this do that comes first. (In Shakespeare English this was typically not the case: "Eats the boy a lemon?" rather than "Does the boy eat a lemon?")
Under certain special circumstances you can also turn a normal sentence into a question buy just sticking a question mark at the end. This is to express a special meaning of the question such as surprise about a fact you just learned. This also works the same way in all Germanic languages. (It is different in French, for example. In French a non-inverted question is just as normal as an inverted one using some form of formulaic periphrasis, and more normal than an inverted one without that.)