Mi volas demandi pri unu afero.

Mi volas demandi means I want to ask.

• volas = want
• demandi = to ask

So this structure is:

• mi = I
• volas = want
• demandi = to ask

If you said mi demandas, that would mean I ask or I am asking, not I want to ask.

This is a very common Esperanto pattern:

• Mi volas iri = I want to go
• Ŝi povas veni = She can come
• Ni devas lerni = We must learn

A conjugated verb comes first, and the second verb stays in the infinitive form ending in -i.

In Esperanto, -i is the infinitive ending, like English to ....

So:

• demandi = to ask
• voli = to want
• paroli = to speak
• esti = to be

In the sentence, volas is the conjugated verb, and demandi stays in the infinitive because it depends on volas.

Compare:

• Mi volas demandi. = I want to ask.
• Mi demandas. = I ask / I am asking.

Pri usually means about, concerning, or regarding.

So:

• demandi pri unu afero = ask about one thing / one matter

This is a very common preposition in Esperanto. For example:

• Ni parolas pri la libro. = We are talking about the book.
• Li demandas pri vi. = He asks about you.

In this sentence, pri introduces the topic of the question.

Yes. In Esperanto, demandi pri io means to ask about something.

That is a normal and common pattern:

• Mi demandas pri la horo. = I ask about the time.
• Ŝi demandis pri via familio. = She asked about your family.

You can also ask a person directly, often with an object:

• Mi demandas vin. = I ask you.
• Mi demandas vin pri la problemo. = I ask you about the problem.

So in your sentence, the person being asked is simply not stated, because it is not necessary in the context.

Unu afero literally means one thing, but in natural English it often feels like one matter, one point, or something.

Using unu can make the phrase feel a bit more specific:

• afero = a thing / matter / issue
• unu afero = one thing / one matter / a certain thing

So Mi volas demandi pri unu afero has the sense of:

• I want to ask about one thing
• There is one matter I want to ask about

If you said Mi volas demandi pri afero, that would sound less natural in most contexts.

Afero is a noun meaning matter, affair, issue, or sometimes thing, depending on context.

It is often a little more specific or abstract than the very broad English word thing.

Examples:

• Tio estas grava afero. = That is an important matter.
• Mi havas unu aferon por diri. = I have one thing to say.

So here, unu afero is not necessarily a physical object. It more likely means one point, one issue, or one matter.

Because unu afero is indefinite.

• la afero = the matter / the thing
• unu afero = one matter / one thing

In Esperanto, la is the definite article, like English the. You use it when the listener is expected to know which thing you mean.

Here, the speaker is just introducing a matter, not referring to a previously identified one, so la is not used.

Also, unu already helps mark it as indefinite and specific in the sense of one or a certain.

Not always. Unu can be a literal number, but it can also mean something like a certain or one particular.

So unu afero can mean:

• literally one thing
• or more naturally one particular matter

This is similar to English, where one sometimes does more than just count:

• There’s one problem.
• One day I’ll visit.

In this sentence, unu probably emphasizes that the speaker has a specific point in mind.

This is the most neutral, standard word order:

• Mi = subject
• volas = main conjugated verb
• demandi = infinitive
• pri unu afero = prepositional phrase

Esperanto word order is fairly flexible, so other orders are possible for emphasis. For example:

• Pri unu afero mi volas demandi.

This puts extra focus on pri unu afero.

But for a learner, the original order is the safest and most natural choice.

It can be implied.

Demandi often involves asking someone, and Esperanto can state that person explicitly:

• Mi volas demandi vin pri unu afero. = I want to ask you about one thing.

But if the listener is obvious from the situation, Esperanto often leaves it out:

• Mi volas demandi pri unu afero.

That sounds perfectly natural if you are already speaking to the person.

This is a very important distinction for English speakers, because English ask can mean two different things.

In Esperanto:

• demandi = ask a question
• peti = ask for / request

So:

• Mi volas demandi pri unu afero. = I want to ask about one thing.
• Mi volas peti helpon. = I want to ask for help.

English uses ask for both ideas, but Esperanto separates them clearly. That is why demandi is the correct word here: the speaker wants to ask about something, not request something.

Yes, in many contexts that is a very natural translation.

Literally, it means I want to ask about one thing, but in real conversation English might prefer:

• I’d like to ask about something.
• I want to ask about one thing.
• There’s one thing I want to ask about.

The Esperanto sentence itself is straightforward and neutral. The best English translation depends on tone and context.