"When does the beginner learn Esperanto?"
Translation:Kiam la komencanto lernas Esperanton?
I translated it as "Kiam lernas la komencanto esperanton?". Case markings make word function clear regardless of word order (in this sentence, anyway).
Why is the "la" not necessary as it would be with Spanish? (e.g. Mi lernas paroli LA hispanan)
La hispana is short for la hispana lingvo, the Spanish language. Without a noun or a pronoun, the adjective preceded by the article is enough to unambiguously imply a language.
Esperanto, on the other hand, is the name of the language itself. It is not the "Esperantan language", and it is capitalised to distinguish it from "esperanto" someone who hopes.
La esperanta lingvo = the hoping language = the language that hopes.
La Esperanta lingvo = the Esperantan language = the language that has to do with Esperanto (without necessarily being Esperanto itself).
Esperanto = Esperanto = the language known as Esperanto.
I believe some Esperantistoj addressed this in the Languages section. If I recall correctly, "Esperanto" was not the original name of this language, so as it became used this way, it became an exception to the language rule of prefacing with "la". One of the few rare exceptions to this language, as it happens!
Many languages are written as la angla (lingvo), la hispana, etc, but Esperanto is the name of the language. You don’t call it the Esperantish language. And even if you did, it would be «la esperantan», not «la esperanta» as it’s the object of the sentence.
Ok but why? The thing is: Esperanto is a very regular language, then why make an exception?
Well, it was never Zamenhof's intention to call the language Esperanto. He called it the international language, "la internacia lingvo", and if that had remained its name, you could have said "la internacia" to mean Esperanto. "Mi lernas la internacian" to mean "I am learning Esperanto".
But people quickly started calling it Esperanto, which stuck because it is a better name for a language, but unlike the other toponyms, it's not connected to a country or a people.
While saying la esperanta to mean the language is actually used, it's not considered good Esperanto, but it might have been considered correct on Duolingo if you had written la esperantan, with the accusative -n. Not all the "correct" possibilities are good, however.
The reason Esperanto is capitalised, by the way, is to disambiguate it from esperanto, which means "someone who hopes".
Because Esperanto is supposed to be perfectly regular, you get a problem when people don't use it correctly, use it inconsistently, or there actually are irregularities in the language. Since it is otherwise highly regular, these inconsistencies stick out like sore thumbs and often become a point of lengthened debate amongst users.
Toponyms is one such subject. Rather than go into the extent it is a problem in this reply, there is a good overview in the answer here: https://esperanto.stackexchange.com/questions/219/why-are-country-names-in-esperanto-so-irregular