La plombaĵo estas en la dento.

In this sentence, plombaĵo means a dental filling.

It is built from:

• plomb- = the root
• -aĵ- = a suffix meaning a concrete thing, substance, or result
• -o = noun ending

So plombaĵo is the actual filling itself.

Esperanto has:

• la = the
• no separate word for a/an

So:

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

If Esperanto leaves out la, the noun can be indefinite or general depending on context. Here, la shows that the speaker has a specific filling and a specific tooth in mind.

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

So La plombaĵo estas en la dento literally has the structure:

• The filling
• is
• in the tooth

A useful point: estas does not change for I/you/he/they the way English am/is/are does. It stays estas for all persons and numbers.

Because this sentence describes location, not motion.

• en la dento = in the tooth / located in the tooth
• en la denton = into the tooth / movement into the tooth

The -n ending is used:

• for direct objects
• and after some prepositions to show movement toward something

Here, nothing is moving. The filling is already there, so dento has no -n.

This is a very common Esperanto pattern:

• en la dento = in the tooth
  Static position.

• en la denton = into the tooth
  Movement toward the inside.

For example:

• La plombaĵo estas en la dento. = The filling is in the tooth.
• La dentisto metas la plombaĵon en la denton. = The dentist puts the filling into the tooth.

A few key points:

• Stress in Esperanto is always on the second-to-last syllable
• ĵ sounds like the s in measure or the g in garage

So plombaĵo is pronounced roughly:

• plom-BA-zho

And:

• dento = DEN-to
• estas = ES-tas

Yes, Esperanto word order is fairly flexible, but this sentence uses the most neutral order:

• La plombaĵo estas en la dento.

You could change the order for emphasis, for example:

• En la dento estas la plombaĵo.

That might sound more like In the tooth is the filling or The filling is in the tooth with special focus on location. For ordinary statements, the original order is the most natural.

Because Esperanto is much simpler than many European languages in this area.

• Verbs do not agree with the subject in person or number
  So it is always estas in the present tense.

• Nouns do not have grammatical gender
  So plombaĵo and dento are just nouns, without masculine/feminine forms.

If you wanted plural, you would add -j:

• La plombaĵoj estas en la dentoj. = The fillings are in the teeth.