La dentisto trovis karion.

La means the.

So la dentisto = the dentist.

If you said just dentisto, it would mean a dentist or simply dentist, depending on context. Esperanto does not have a separate word for a/an, so the absence of la often gives that indefinite sense.

• la dentisto = the dentist
• dentisto = a dentist / dentist

In Esperanto, all nouns end in -o.

So:

• dentisto is a noun
• kario is a noun

This is one of the most regular parts of Esperanto grammar. When you see -o, you can usually immediately recognize the word as a noun.

Examples:

• dento = tooth
• dentisto = dentist
• kario = cavity / dental caries

The -n marks the direct object.

In this sentence, the dentist is doing the finding, and the thing found is the cavity. So kario is the direct object, and it becomes karion.

• La dentisto trovis karion.
• The dentist found a cavity.

A useful way to think about it is: found what? → karion

That is why it gets -n.

Because dentisto is the subject, not the direct object.

The subject is the one performing the action:

• la dentisto = the one who found
• karion = the thing that was found

In Esperanto, the subject normally has no special ending, while the direct object takes -n.

Trovis is past tense.

In Esperanto, verb endings are very regular:

• -as = present
• -is = past
• -os = future

So:

• trovas = finds / is finding
• trovis = found
• trovos = will find

That means La dentisto trovis karion is talking about something that already happened.

Yes.

Dentisto can be analyzed as:

• dent- = tooth-related root
• -ist- = a person who works in a profession or practice
• -o = noun ending

So dentisto literally means something like tooth-professional.

Related words:

• dento = tooth
• dentisto = dentist

This kind of word-building is very common in Esperanto.

Kario is a dental term meaning caries, often translated in ordinary English as a cavity or tooth decay, depending on context.

So in a learner-friendly sense, karion here is best understood as a cavity.

Because the sentence meaning has already been given, the main grammar point is that it is a noun in the direct object form:

• base form: kario
• object form: karion

Yes. Because Esperanto marks the direct object with -n, word order is more flexible than in English.

For example, these can mean the same thing:

• La dentisto trovis karion.
• Karion trovis la dentisto.

Both still mean that the dentist found a cavity, because karion has the object marker -n.

That said, subject–verb–object order is the most neutral and common, so La dentisto trovis karion is the most straightforward version.

Yes, kario is singular.

To make a noun plural in Esperanto, add -j:

• kario = a cavity
• karioj = cavities

If the plural noun is also a direct object, it takes both endings:

• kariojn = cavities (as a direct object)

Example:

• La dentisto trovis kariojn. = The dentist found cavities.

No. La is always la.

It does not change for:

• singular or plural
• subject or object
• gender

So you get:

• la dentisto = the dentist
• la dentistoj = the dentists
• la karion would be wrong, because the noun changes, not la
• la kario / la karion = the cavity / the cavity (as object)

This is simpler than in many languages, since the article stays the same in every situation.