Miliono da homoj konas tiun artiston.

Because miliono is a normal Esperanto noun meaning a million.

• miliono = one million
• milionoj = millions

In this sentence, the subject is one million of people, so the singular miliono is correct.

Examples:

• Miliono da homoj venis. = A million people came.
• Du milionoj da homoj venis. = Two million people came.

After a word showing quantity, Esperanto normally uses da.

So:

• miliono da homoj = a million people
• glaso da akvo = a glass of water
• multe da tempo = a lot of time

de usually means of in a different sense, or from, and would not be the normal choice for a quantity expression here.

A useful rule:

• use da when you are talking about how much/how many
• use de for other relationships, such as possession or origin

Because homoj refers to the actual people, and there are many of them.

Even though miliono is grammatically singular, it means a quantity containing many individuals. So the noun after da is plural when the meaning is plural:

• miliono da homoj = a million people
• cento da libroj = a hundred books

You would not normally say miliono da homo here, because the meaning is clearly more than one person.

Because in a da phrase, the case ending goes on the quantity word if needed, not on the noun after da.

Here, miliono da homoj is the subject, so no accusative is needed at all.

Compare:

• Miliono da homoj konas tiun artiston. = A million people know that artist.
• Mi vidis milionon da homoj. = I saw a million people.

Notice in the second sentence:

• milionon gets -n
• homoj stays the same

So the noun after da does not take the accusative ending in this structure.

Because tiun artiston is the direct object of konas, and both the adjective-like word and the noun must agree in case.

• tiu artisto = that artist
• tiun artiston = that artist as a direct object

In Esperanto, modifiers agree with the noun in:

• number
• case

So if the noun has -n, the demonstrative also gets -n.

Tiun is the accusative singular form of tiu, which means that or sometimes that one / the one in question.

Forms:

• tiu = that
• tiun = that, as a direct object
• tiuj = those
• tiujn = those, as direct objects

So:

• tiu artisto = that artist
• tiun artiston = that artist, as the thing/person being known

Because Esperanto usually distinguishes between knowing a person/thing and knowing a fact.

• koni = to know, be familiar with, recognize
• scii = to know a fact, to know that something is true

So:

• Mi konas tiun artiston. = I know that artist.
• Mi scias, ke li estas artisto. = I know that he is an artist.

Since the object here is a person, konas is the right verb.

Yes. Esperanto word order is fairly flexible because the accusative -n shows the direct object clearly.

The normal order here is:

• Miliono da homoj konas tiun artiston.

But you could also say:

• Tiun artiston konas miliono da homoj.

That would still mean the same thing, though it may sound more emphatic, especially focusing on that artist.

So Esperanto allows movement of parts of the sentence more easily than English.

Because tiun already makes the noun definite.

In Esperanto, tiu works like that in English, so you do not add la with it.

Correct:

• tiu artisto
• tiun artiston

Not correct:

• la tiu artisto
• la tiun artiston

So tiun already does the job of identifying which artist is meant.

It can be either, depending on context.

By itself, miliono da homoj can mean:

• exactly one million people
• roughly a million people

If the speaker wants to be more precise, Esperanto can add words like:

• ĝuste miliono da homoj = exactly a million people
• proksimume miliono da homoj = approximately a million people

So the basic sentence does not force a precise interpretation unless the context does.