La aĉetanto petas la ŝanĝmonon.

The -n ending marks the direct object in Esperanto.

• La aĉetanto = the buyer → the subject, the person doing the action
• petas = asks for
• la ŝanĝmonon = the change → the thing being asked for

So ŝanĝmonon gets -n because it is what the buyer is asking for.

A very literal grammatical breakdown is:

• La aĉetanto = subject
• petas = verb
• la ŝanĝmonon = direct object

The ending -anto is an active participle used as a noun-like form meaning someone who is doing the action of the verb.

Here:

• aĉeti = to buy
• aĉetanto = a buyer, a person who buys / is buying

So aĉetanto is built from the verb aĉeti plus -anto.

This is a very common pattern in Esperanto:

• lernanto = learner, student
• parolanto = speaker
• laboranto = worker

Because -as is the present-tense verb ending in Esperanto.

• peti = to ask for
• petas = asks for / is asking for

In Esperanto, verb endings are very regular:

• -i = infinitive → peti = to ask
• -as = present → petas = asks
• -is = past → petis = asked
• -os = future → petos = will ask

So in a full sentence, you normally use petas, not peti.

Because both nouns are definite.

• La aĉetanto = the buyer
• la ŝanĝmonon = the change

In Esperanto, if both nouns are definite, each one normally gets its own la. Esperanto does not usually skip the article the way English sometimes can.

So this is completely normal:

• La knabo vidas la hundon.
• La instruisto demandas la lernanton.

Each definite noun phrase has its own la.

Yes. ŝanĝmono is a compound.

It is made from:

• ŝanĝo = change
• mono = money

So ŝanĝmono literally means change-money, that is, the money returned after paying.

This kind of compound is very common in Esperanto. Usually the last part is the main idea, and the earlier part modifies it.

So:

• mono = money
• ŝanĝmono = money given as change

Usually peti means to ask for or to request something.

That is important, because English ask can mean two different things:

• ask a question
• ask for something

Esperanto often separates these more clearly:

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

So in this sentence, the buyer is not asking a question. The buyer is requesting the change, so petas is the right verb.

Yes, to some extent.

Because Esperanto marks the direct object with -n, the roles are clear even if the order changes. For example:

• La aĉetanto petas la ŝanĝmonon.
• La ŝanĝmonon petas la aĉetanto.

Both are grammatically possible. However, the first version is the most neutral and natural in ordinary usage: subject + verb + object.

So a learner should usually start with:

• La aĉetanto petas la ŝanĝmonon.

The special letters here are ĉ and ŝ.

• ĉ is pronounced like ch in church
• ŝ is pronounced like sh in shoe

So:

• aĉetanto starts with a sound like ah-che-
• ŝanĝmonon starts with sh

Also, Esperanto spelling is very regular: one letter normally represents one sound.

Because singular nouns in Esperanto normally just use the basic noun ending -o.

• aĉetanto = buyer (singular)
• aĉetantoj = buyers (plural)

Plural is marked with -j, not singular. Singular is the default.

So:

• la aĉetanto = the buyer
• la aĉetantojn = the buyers, if they were direct objects

Not necessarily wrong in all contexts, but it changes the feel.

• la ŝanĝmonon = the change, specific change expected from a payment
• ŝanĝmonon = change / some change, less specifically marked as definite

In many everyday situations, la ŝanĝmonon sounds natural because the buyer is asking for a specific amount of change that should be returned.

So the version with la is a good standard sentence for learners.