La artikolo pri la urbo estas interesa.

Because there are two separate noun phrases that are definite:

• la artikolo = the article
• la urbo = the city

Esperanto uses la for the, and you use it wherever English would mean something definite. So the article about the city naturally has la before both nouns.

Pri means about, concerning, or regarding.

So:

• artikolo pri la urbo = an article about the city

It introduces the topic of the article. It is a preposition, and in Esperanto prepositions usually determine the relationship clearly by themselves, so you normally do not add -n after them.

Estas is the present-tense form of esti, which means to be.

A very important Esperanto feature is that verbs do not change according to the subject:

• mi estas = I am
• vi estas = you are
• li estas = he is
• ili estas = they are

So estas always means am / is / are, depending on context.

Because interesa is an adjective, and adjectives in Esperanto end in -a.

Here it means interesting. It describes la artikolo.

Even after estas, the adjective still keeps the adjective ending:

• La artikolo estas interesa. = The article is interesting.

If the noun were plural, the adjective would usually match it:

• La artikoloj estas interesaj. = The articles are interesting.

Because there is no direct object here.

Breakdown:

• La artikolo is the subject.
• estas is the verb.
• interesa is a predicate adjective.
• pri la urbo is a prepositional phrase.

The accusative -n is mainly used for direct objects and for some expressions of direction. In this sentence:

• la urbo does not take -n because it follows the preposition pri
• interesa does not take -n because it is not a direct object

In the normal reading, it describes artikolo.

So the structure is:

• La artikolo pri la urbo = The article about the city
• estas interesa = is interesting

That is the most natural interpretation because pri la urbo comes right after artikolo, so it attaches to that noun phrase.

Not completely. Esperanto word order is fairly flexible, but this sentence uses the most neutral order.

Normal structure:

• La artikolo pri la urbo = subject
• estas = verb
• interesa = complement

This is the most straightforward way to say it.

You could change the order for emphasis, but the meaning or style may shift slightly. For example:

• Interesa estas la artikolo pri la urbo.

That is possible, but less neutral and more emphatic.

Yes, but it would change the meaning.

• pri la urbo = about the city (a specific city)
• pri urbo = about a city or about city / urban matters, depending on context

In your sentence, la urbo suggests a particular city that both speaker and listener can identify.

Yes. Esperanto has only one article: la, which is the definite article.

There is no separate word for a or an. Indefinite meaning is usually shown by simply leaving out the article:

• artikolo = an article / article
• la artikolo = the article

So Esperanto is simpler than English in this area.

Because interesa is linked through estas to the subject, which is la artikolo, not la urbo.

The sentence says that the article is interesting, not that the city is interesting.

So:

• La artikolo ... estas interesa = the article is interesting

If you wanted to say the city is interesting, you would say:

• La urbo estas interesa.

Yes, adjectives can come before the noun in Esperanto.

So you can say:

• la interesa artikolo = the interesting article

But that is a different structure from your original sentence:

• La artikolo estas interesa. = The article is interesting.
• La interesa artikolo... = The interesting article...

Both are correct, but one uses an adjective inside the noun phrase, while the other uses a predicate adjective after estas.