Estas necese, ke la dentisto kontrolu la ŝvelaĵon.

Both are possible.

• Estas necese literally means It is necessary.
• Necesas means It is necessary / there is a need.

So:

• Estas necese, ke la dentisto kontrolu la ŝvelaĵon.
• Necesas, ke la dentisto kontrolu la ŝvelaĵon.

Both are natural. The version with estas necese is a little more explicitly built like English, while necesas is a compact Esperanto alternative.

Because necese is being used in an impersonal expression: Estas necese = It is necessary.

In Esperanto:

• necesa is an adjective, used with a noun
  
  • La kontrolo estas necesa. = The check-up is necessary.
• necese is an adverb, used in expressions like estas necese
  
  • Estas necese kontroli... = It is necessary to check...

So here there is no noun being described directly by necesa, which is why necese is used.

Ke introduces a subordinate clause, like that in English.

So:

• Estas necese = It is necessary
• ke la dentisto kontrolu la ŝvelaĵon = that the dentist check the swelling

In English, that is often optional, but in Esperanto ke is normally stated clearly.

Because after expressions like Estas necese, ke..., Esperanto usually uses the -u verb form.

The -u form is used for:

• commands
• wishes
• recommendations
• necessities
• intended actions

Here it expresses what should or needs to happen:

• la dentisto kontrolu = the dentist should check / is to check

Compare:

• La dentisto kontrolas la ŝvelaĵon. = The dentist is checking/checks the swelling.
  This states a fact.
• La dentisto kontrolos la ŝvelaĵon. = The dentist will check the swelling.
  This states a future fact.
• Estas necese, ke la dentisto kontrolu la ŝvelaĵon. = It is necessary that the dentist check the swelling.
  This expresses necessity, not simple fact.

Not directly in this sentence.

By itself, Kontrolu la ŝvelaĵon! would be a direct command: Check the swelling!

But in Estas necese, ke la dentisto kontrolu la ŝvelaĵon, the -u form appears inside a ke-clause after an expression of necessity. So it means something like:

• the dentist should check
• the dentist needs to check
• it is necessary for the dentist to check

So this is not the speaker ordering the dentist face-to-face; it is a statement about what needs to happen.

La is the definite article, meaning the.

So la dentisto means the dentist.

Esperanto uses la when the person or thing is specific, identifiable, or already understood from context. In a real situation, this could mean:

• the dentist treating the patient
• the relevant dentist in the discussion
• the dentist already mentioned earlier

If you said dentisto without la, it would sound more like a dentist or any dentist, depending on context.

The -n marks the direct object.

The verb kontroli acts on something, and that thing gets -n:

• La dentisto kontrolu la ŝvelaĵon.
• The dentist should check what?
• la ŝvelaĵon

So ŝvelaĵon is in the accusative because it is the thing being checked.

The base dictionary form is ŝvelaĵo.

It breaks down like this:

• ŝvel- = swell
• -a = adjective ending, giving the idea swollen
• -aĵ- = a concrete thing, manifestation, or substance
• -o = noun ending

So ŝvelaĵo means something like:

• a swelling
• a swollen area
• swollen tissue

Then -n is added in the sentence because it is the direct object:

• ŝvelaĵo → ŝvelaĵon

Yes, very often Esperanto can use an infinitive construction here:

• Estas necese, ke la dentisto kontrolu la ŝvelaĵon.
• Estas necese por la dentisto kontroli la ŝvelaĵon.

However, the ke + -u construction is especially common when you want to clearly state who is supposed to do the action.

So:

• ke la dentisto kontrolu clearly focuses on the dentist as the one who should act
• por la dentisto kontroli is also possible, but can feel a bit more structural or less direct depending on context

The original sentence is very natural.

Not completely. Esperanto word order is fairly flexible, although some orders sound more neutral than others.

The given sentence is the most straightforward:

• Estas necese, ke la dentisto kontrolu la ŝvelaĵon.

You could also rearrange parts for emphasis, for example:

• Ke la dentisto kontrolu la ŝvelaĵon, estas necese.

That is grammatical, but less neutral and more emphatic or literary. For normal use, the original order is best.

Esperanto often expresses ideas like should, must, or needs to through structure rather than a direct one-word equivalent.

In this sentence, the combination of:

• Estas necese = it is necessary
• ke ... kontrolu = that ... do/check

naturally gives the meaning of should / needs to.

So instead of a single modal verb like English should, Esperanto often uses:

• estas necese, ke...
• devas
• necesas, ke...

Each has a slightly different tone, but all can express obligation or necessity.

Yes, but it changes the structure and slightly changes the tone.

For example:

• La dentisto devas kontroli la ŝvelaĵon.

This means The dentist must/has to check the swelling.

Compared with the original:

• Estas necese, ke la dentisto kontrolu la ŝvelaĵon.

The original sounds a bit more impersonal and formal: it states that the action is necessary. The devas version sounds more direct: it says the dentist has an obligation.

Both can be correct depending on context.

It is a verb in the volitive form, marked by -u.

This form is used for:

• commands
• requests
• wishes
• recommendations
• necessity statements in subordinate clauses

Examples:

• Venu! = Come!
• Li venu morgaŭ. = Let him come tomorrow.
• Mi volas, ke ŝi venu. = I want her to come.
• Estas necese, ke la dentisto kontrolu la ŝvelaĵon.

So in this sentence, kontrolu is not present tense or future tense; it is the -u form used because the action is desired or required.