Dum la dentokontrolo la dentisto faris rentgenbildon.

Because it is the direct object of faris.

In Esperanto, the direct object usually takes the accusative ending -n. Here:

• la dentisto = the dentist → the subject, the one doing the action
• faris = did/made
• rentgenbildon = an X-ray image → the thing that was made/taken

So rentgenbildo becomes rentgenbildon because it is what the dentist made.

Because la dentisto is the subject, not the object.

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

So in this sentence:

• la dentisto = the dentist did the action
• rentgenbildon = the X-ray image received the action

That is why only rentgenbildon has -n.

Dum means during or while.

In this sentence, dum la dentokontrolo means during the dental checkup.

A useful point:

• dum + noun phrase = during
• dum + clause = while

Examples:

• dum la leciono = during the lesson
• dum mi legis = while I was reading

Here it is followed by la dentokontrolo, a noun phrase, so it means during.

It is one compound word, and compounds are very common in Esperanto.

It is built from:

• dento = tooth
• kontrolo = check, inspection

So dentokontrolo literally means something like tooth-check, that is, a dental checkup or dental examination.

In Esperanto, the last part is usually the main idea, and the earlier part modifies it:

• vaporŝipo = steamship
• lernejo = learning-place, school
• dentokontrolo = tooth-check

La is the definite article, meaning the.

So:

• la dentokontrolo = the dental checkup
• la dentisto = the dentist

Esperanto has only one definite article: la. It does not change for gender, number, or case.

You use la when the speaker has a specific thing or person in mind, just like the in English.

Faris is the past tense of fari, which means to do or to make.

Esperanto verb endings are very regular:

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

So:

• faras = does/makes
• faris = did/made
• faros = will do/make

Here faris is used because the action happened in the past.

Yes, in English we usually say take an X-ray, but Esperanto often uses expressions that are not exact word-for-word matches with English.

fari rentgenbildon literally looks like make an X-ray image, but it is a normal way to express the idea that the dentist produced or took an X-ray image.

Languages often package actions differently. So even if English prefers take, Esperanto may use fari naturally in this context.

Depending on context, other expressions may also exist, but faris rentgenbildon is understandable and structurally straightforward.

Rentgenbildo means X-ray image.

It is made from:

• Rentgeno / rentgen- = X-ray, relating to Röntgen
• bildo = image, picture

So rentgenbildo is literally X-ray picture/image.

You may also notice that Esperanto often forms technical words this way, by combining meaningful parts into one compound.

Yes, Esperanto word order is fairly flexible, because the grammar endings show what each word is doing.

For example, these are all possible:

• Dum la dentokontrolo la dentisto faris rentgenbildon.
• La dentisto faris rentgenbildon dum la dentokontrolo.

Both mean essentially the same thing. The original version puts the time expression first, which gives a little more emphasis to during the checkup.

Even with flexible word order, the most common style is still fairly clear and natural, often similar to English.

Yes. It tells you when the action happened.

The core sentence is:

• la dentisto faris rentgenbildon

Then dum la dentokontrolo adds time information:

• during the dental checkup

So it functions as a time expression or adverbial phrase.

Possibly, yes. Esperanto often allows more than one natural way to say something.

For example, depending on context, speakers might use:

• dentokontrolo
• denta kontrolo
• another phrase that emphasizes the examination or appointment

But dentokontrolo is a very typical Esperanto-style compound and is easy to understand.

A useful lesson for learners is that Esperanto often prefers compact compounds where English might use a multi-word phrase.

A rough English-friendly pronunciation would be:

doom lah den-to-kon-TRO-loh lah den-TIS-toh FAH-ris rent-GEN-bil-don

A few helpful points:

• u sounds like oo
• e is like e in get, but often a bit clearer
• o is a pure o sound
• stress is always on the second-to-last syllable

So:

• dentokontrolo → den-to-kon-TRO-lo
• dentisto → den-TIS-to
• rentgenbildon → rent-gen-BIL-don

That regular stress pattern is one of the nice features of Esperanto.