Mi volas tiun, kiu estas sur la tablo.

Because tiun is the direct object of volas.

• Mi volas = I want
• What do I want? tiun = that one

In Esperanto, a direct object usually takes -n, so:

• tiu = that one
• tiun = that one as the object

So Mi volas tiun literally means I want that one.

Because kiu is the subject of the relative clause kiu estas sur la tablo.

Inside that clause:

• kiu = who/which/that
• estas = is
• sur la tablo = on the table

So the structure is basically:

• that one, which is on the table

Since kiu is the one doing the being, it is the subject, so it stays kiu, not kiun.

Compare:

• tiun, kiu estas sur la tablo = that one, which is on the table
• tiun, kiun mi vidas = that one, which I see

In the second example, kiun is the object of vidas, so it gets -n.

Here kiu is a relative pronoun. It refers back to tiun and connects the extra description.

So:

• tiun = that one
• kiu estas sur la tablo = which is on the table

In natural English, this can be translated as:

• I want the one that is on the table
• I want that one, which is on the table

Depending on context, kiu can correspond to English who, which, or that.

Because kiu is used for which one / who and also as a relative pronoun referring to a specific thing or person already mentioned.

Here, kiu points back to tiun, so it means something like the one which or the one that.

kio usually means what and does not normally refer back to a noun or pronoun in this way.

So:

• kiu = which one / who / that which
• kio = what

That is why tiun, kiu estas sur la tablo is correct, not tiun, kio estas sur la tablo.

By itself, tiun most naturally means that one.

It does not name the object directly; it points to it. English often does the same:

• I want that one.

So in this sentence, Esperanto is not repeating a noun like book or cup. It simply uses the demonstrative pronoun tiun.

Context tells you what kind of thing it is.

Yes.

In a sentence like this, kiu can refer to:

• a person: the person who...
• a thing: the thing which...
• an animal: the one that...

So Esperanto does not require a different relative pronoun for who versus which in the way English often does. Kiu can cover both.

Because Esperanto normally uses a comma before a subordinate clause, including a relative clause introduced by kiu.

So:

• Mi volas tiun, kiu estas sur la tablo.

This is standard punctuation in Esperanto.

English is often less consistent about commas in clauses like this, but Esperanto tends to mark them more regularly.

Because this sentence describes location, not movement.

• sur la tablo = on the table (already there)
• sur la tablon = onto the table (movement toward the table)

Here the meaning is that the thing is on the table, so no accusative -n is needed after sur.

Compare:

• Ĝi estas sur la tablo. = It is on the table.
• Mi metas ĝin sur la tablon. = I put it onto the table.

La means the.

So sur la tablo means on the table, not just on a table.

Using la usually suggests that the speaker and listener can identify which table is meant, or that the table is clear from context.

This word order is the most natural and straightforward:

• Mi volas tiun, kiu estas sur la tablo.

Because Esperanto marks the direct object with -n, word order is somewhat flexible, but not all variations sound equally natural.

This sentence is a very standard way to say it. A learner should treat it as the normal pattern.

No noun has to be stated. Tiun can stand on its own.

Esperanto often allows demonstratives and other pronouns to replace a noun when the meaning is clear.

So this is perfectly normal:

• Mi volas tiun. = I want that one.

If you wanted, you could also include a noun:

• Mi volas tiun libron, kiu estas sur la tablo. = I want that book which is on the table.

But in your sentence, the noun is simply left unstated because context already supplies it.

Usually Esperanto uses kiu where English might use who, which, or that.

So in this sentence, English could say:

• the one that is on the table
• the one which is on the table

Esperanto normally uses kiu for all of these. So yes, kiu often does the job that English that does in relative clauses.