La dento apud la plombaĵo estas sentema.

Because both dento and plombaĵo are being treated as specific things.

• la dento = the tooth
• la plombaĵo = the filling

Esperanto uses la much like English the. If both the tooth and the filling are known or identifiable in context, it is natural to use la for both.

Apud means next to, beside, or adjacent to.

So la dento apud la plombaĵo means the tooth that is next to the filling.

It is a preposition showing physical closeness. In this sentence, it helps identify which tooth is being talked about.

Because plombaĵo means something like a filling or a thing made/used in filling.

This comes from:

• plomb- = the root related to filling a tooth / plomb
• -aĵ- = a suffix meaning a concrete thing, material thing, or result/product
• -o = noun ending

So plombaĵo is the actual filling itself.

For learners, -aĵ- is a very useful suffix to notice: it often turns an idea or action into a physical thing.

Sentema means sensitive.

It is built from:

• sent- = feel
• -em- = inclined to, prone to, tending to
• -a = adjective

So literally it suggests something like prone to feeling strongly or easily affected by sensation.

In a dental context, sentema is a very natural way to say that a tooth is sensitive.

Because it is an adjective, not a noun.

In Esperanto:

• -o = noun
• -a = adjective
• -e = adverb

Here, sentema describes dento, so it must be an adjective.

Even after estas, Esperanto still uses the adjective form:

• La dento estas sentema. = The tooth is sensitive.

Just like in English, the word after is can still be an adjective.

Because nothing in this sentence is a direct object, and there is no motion toward something.

The sentence is just:

• subject: La dento
• prepositional phrase: apud la plombaĵo
• predicate: estas sentema

The accusative -n is usually used for:

1. a direct object
2. motion toward a place in some expressions

Neither applies here, so no -n is needed.

Because the main subject of the sentence is La dento.

The structure is:

• La dento = the subject
• apud la plombaĵo = extra information identifying which tooth
• estas sentema = tells us something about the subject

So the sentence says that the tooth is sensitive. The phrase apud la plombaĵo only tells you which tooth.

Yes, Esperanto word order is fairly flexible, though some orders sound more neutral than others.

The most neutral version is:

• La dento apud la plombaĵo estas sentema.

You could rearrange parts for emphasis, for example:

• Sentema estas la dento apud la plombaĵo.

But that sounds more marked or literary. For learners, the original order is the safest and most natural.

Estas is the present-tense form of esti, meaning to be.

So:

• estas = is / are / am, depending on context

Esperanto verbs do not change according to person or number:

• mi estas
• vi estas
• li estas
• la dento estas

The verb form stays the same.

Yes, dento is singular because it ends in -o.

To make it plural, use -j:

• dento = tooth
• dentoj = teeth

So The teeth next to the filling are sensitive would be:

• La dentoj apud la plombaĵo estas sentemaj.

Notice that the adjective also becomes plural:

• sentema → sentemaj

In Esperanto, adjectives agree with the nouns they describe.

A simple pronunciation guide is:

La DEN-to a-PUD la plom-BA-jo ES-tas sen-TE-ma.

A few helpful points:

• stress is usually on the second-to-last syllable
• ĵ sounds like the s in measure
• every written vowel is pronounced clearly

So plombaĵo is roughly plom-BA-zho.

Sometimes, yes, but next to or beside is usually clearer.

English by can mean many things, so for learning purposes it is better to connect apud with:

• beside
• next to
• adjacent to

That will help avoid confusion with other Esperanto prepositions.