La rentgenbildo montras ne nur la karion, sed ankaŭ la kronon.

Because they are direct objects of montras.

In Esperanto, a direct object normally takes the accusative ending -n. The dictionary forms are kario and krono. In the sentence, they become:

• kario → karion
• krono → kronon

Since the X-ray image is showing both things, both nouns are objects, so both get -n.

Montras is the present-tense form of the verb montri, meaning to show.

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command / jussive
• -i = infinitive

So montras means shows / is showing.

Also, Esperanto verbs do not change for person or number. So:

• mi montras
• vi montras
• li montras
• ili montras

all use montras.

It is the noun phrase without the accusative -n, and it comes before the verb in the normal word order.

So:

• La rentgenbildo = the subject
• montras = the verb
• la karion and la kronon = the direct objects

Because Esperanto marks direct objects with -n, it is often easier to see who is doing the action and what is receiving it than in English.

Because Esperanto commonly forms compound words by joining roots together into a single word.

Here:

• rentgen- = X-ray
• bildo = image, picture

So rentgenbildo literally means X-ray image.

The final part of the compound determines the grammatical category. Since bildo is a noun, rentgenbildo is also a noun.

Writing it as one word is normal Esperanto style.

This is the standard Esperanto pattern for not only ... but also ....

In this sentence:

• ne nur la karion
• sed ankaŭ la kronon

The structure links two parallel parts. Here, the two linked parts are both direct objects.

A very useful thing to notice is that Esperanto likes these paired expressions to be structurally parallel. Since la karion is a noun phrase, la kronon is also a noun phrase.

Because sed ankaŭ is the normal partner to ne nur in this construction.

So the full pattern is:

• ne nur ... sed ankaŭ ...

Here, ankaŭ helps mark the second item as the added one. It comes before the element it emphasizes, in this case la kronon.

So:

• sed ankaŭ la kronon = but also the crown

This placement is the most natural one in this kind of sentence.

Because each noun phrase is definite in its own right.

So Esperanto naturally says:

• la karion
• la kronon

Repeating la is especially natural when two items are contrasted or balanced, as they are in ne nur ... sed ankaŭ ....

If you removed the articles, the sentence would sound less definite, more like talking about a cavity and a crown in general rather than specific ones.

Yes.

Krono literally means crown, and context determines which kind. In a dental context, it can mean a dental crown.

Kario refers to caries / tooth decay, and in ordinary learner-friendly translation it is often understood as a cavity.

So both words are perfectly normal here in a dental or medical context.

Yes, Esperanto word order is fairly flexible, but the original order is very clear and natural.

The given sentence puts the contrast directly on the two objects:

• ne nur la karion, sed ankaŭ la kronon

That makes it especially easy to see that the X-ray shows both of those things.

If you moved ne nur somewhere else, the sentence could still be grammatical, but the focus might shift slightly. The original is a good, straightforward model.

Esperanto normally stresses the second-to-last syllable.

So here you get:

• rentgenbildo → ren-tgen-BIL-do
• montras → MON-tras
• karion → ka-RI-on
• ankaŭ → AN-kaŭ
• kronon → KRO-non

A few extra pronunciation notes:

• g is always hard, like in go
• r is usually tapped or rolled
• aŭ is a diphthong, roughly like ow in cow

So even though rentgenbildo looks long, its pronunciation is regular once you split it into syllables.