"The teacher asks the boys about something."
Translation:La instruisto demandas la knabojn pri io.
"Al" is not really optional. Rather, there are two ways of marking the role that "la knaboj" and "io" have in this sentence.
- demandi la knabojn pri io.
demandi al la knaboj ion.
Ask the boys something.
- As something of the boys.
- Ask the boys about something.
"La instruisto demandas al la knaboj pri io.", "La instruisto demandas la knabojn pri io."
Some prepositions can be replaced by the accusation-ending, if that doesn't introduce ambiguity, or make the sentence have two direct objects.
I think that "La instruisto demandas ion al la knaboj" is also possible. For me it makes sense and it's the same contruction as "I ask him a question". Am I right?
That's not quite the same meaning. In the original sentence, the teacher is asking the boys about something. (i.e. What can you tell me about horses?) In your sentence, the teacher is merely asking the boys something. (i.e. How many legs does a horse have?)
I would describe this as "technically possible" - but it's nothing you'd see in practice. The only sentence I found that was even close was something like:
La instruisto demandas al la knaboj nur tion, ĉu ili faris la hejmtaskojn.
The teacher asks the boys only whether they'd done their homework.
In that case "tion" is added to make the function of "nur" more clear. Otherwise, you'd simply say
- La instruisto demandis al la knaboj ĉu ili faris le hejmtaskojn.
Thank you. I had forfotten that and kept wondering why "la knabojn" was accusarive.
So does "demandi" take a direct object of the question being asked or the person being asked...?
Well, re-reading my own answer all these months later, I'm not sure. :-)
The normal pattern is demandi iun pri io -- that is, the direct object is the person asked.
You will also see demandi al iu....
And in questions, the direct object is often the thing askd. "Kion li demandis?"
Properly, demandi is a transitive verb and requires an object. It is unlike the English verb "ask," which is both transitive and intransitive. Moreover, "ask" in English has mutliple meanings and can take different kinds of objects. Some examples: (1) put a question to; inquire of; "I asked her." (2) request information about; "Ask the way." (3) call for; need; "This experiment asks patience."
The Esperanto verb "demandi" has only meaning (1) of "ask" and "demandi al" is a corruption from outside Esperanto.
http://vortaro.net/#demandi - demand/i (tr) Peti pri sciigo, informo: demandi iun pri io
@BillEverett: Your answer seems definitive, but how do I know that it is? You write that "'demandi al' is a corruption from outside Esperanto" Is that definitive? I'd be really grateful to know, either by consensus or unilateral edict, whether that entire quoted statement is true.
Or, as AustinSegal so brilliantly asked, does "demandi" take a direct object?
Thanks in advance for any cogent answers.