Estas necese, ke la paciento revenu por dentokontrolo.

In Esperanto, estas necese is an impersonal expression meaning it is necessary.

The it in English is just a dummy subject, but Esperanto often leaves that out. So:

• Estas necese = It is necessary
• Estas grave = It is important
• Estas bone = It is good

You usually do not need ĝi here.

Necese is an adverb, while necesa is an adjective.

In Estas necese, the word describes the whole situation, not a noun. So Esperanto uses the adverb:

• Estas necese, ke... = It is necessary that...

By contrast, necesa would describe a noun:

• Tio estas necesa = That is necessary
• Necesa kontrolo = a necessary checkup

So in this sentence, necese is the normal choice.

Ke introduces a subordinate clause, usually equivalent to English that.

So:

• Estas necese, ke la paciento revenu... = It is necessary that the patient return...

The part after ke is the thing that is necessary.

This is one of the most common uses of ke in Esperanto.

Revenu is in the -u verb form, which is used for will, desire, necessity, requests, commands, and similar ideas.

After expressions like estas necese, ke..., Esperanto normally uses -u:

• Estas necese, ke li venu. = It is necessary that he come.
• Mi volas, ke vi restu. = I want you to stay.

So revenu here means something like should return or return in the sense of requirement.

Why not the others?

• revenas = returns / is returning
  This would describe a fact, not a requirement.
• reveni = infinitive, to return
  That does not fit after ke in this structure.

Yes, that is a helpful comparison.

In English, some speakers might say:

• It is necessary that the patient return

That return is a subjunctive-like form. Esperanto uses -u much more regularly and clearly for this kind of idea:

• ke la paciento revenu

So if you are used to English grammar terms, you can think of this as one of the main places where Esperanto uses something similar to the subjunctive.

La paciento means the patient, referring to a specific patient already understood in the context.

Esperanto often uses la when the listener can identify the person or thing being talked about. In a medical context, that is very natural.

If you said paciento without la, it would sound more like a patient or patient as a general idea, depending on context.

Yes, reveni means to come back, to return, or to go back again, depending on context.

It is built from:

• re- = again, back
• veni = to come

So:

• reveni = to come back / return
• revenu = return / should return / let them return

In this sentence, it means the patient should come back for a dental checkup.

Dentokontrolo is a compound noun:

• dento = tooth
• kontrolo = check, inspection, examination

Together, dentokontrolo means dental checkup, tooth check, or dental examination, depending on context.

Esperanto very often forms compounds as one word. That is completely normal and very common.

Yes, you could, and it would be understandable.

The difference is mainly stylistic:

• dentokontrolo = a compact compound, very typical Esperanto style
• denta kontrolo = a dental check / dental inspection, using an adjective

Often Esperanto prefers compounds when they are clear and natural. So dentokontrolo sounds very normal here.

Por means for, expressing purpose.

So:

• revenu por dentokontrolo = return for a dental checkup = return with the purpose of having a dental checkup

If you used al, that would mainly indicate movement to a place or destination. But dentokontrolo is an event or purpose here, not really a destination.

So por is the natural choice.

Yes, a comma before a ke-clause is normal Esperanto punctuation.

So:

• Estas necese, ke la paciento revenu...

This helps mark the subordinate clause clearly. You will often see commas before ke, ĉar, se, and similar conjunctions.

Yes. A few natural alternatives are:

• Necesas, ke la paciento revenu por dentokontrolo.
• Estas necese, ke la paciento revenu al dentisto por kontrolo.
• La paciento devas reveni por dentokontrolo.

These are similar, but not identical in tone:

• Estas necese, ke... = it is necessary that...
• Necesas, ke... = same idea, slightly more compact
• devas = must / has to, more direct

The original sentence is formal, natural, and very suitable in medical or administrative language.