Mi aŭdas bruon el la najbara apartamento.

Because bruon is the direct object of the verb aŭdas (hear).

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

• Mi aŭdas bruon = I hear a noise
• Mi aŭdas muzikon = I hear music
• Mi aŭdas la hundon = I hear the dog

So in this sentence, bruo becomes bruon because it is the thing being heard.

Because el la najbara apartamento is a prepositional phrase, not the direct object.

The preposition el means out of / from inside, and nouns after normal prepositions do not take -n just for being after the preposition.

So:

• bruon gets -n because it is the object of aŭdas
• la najbara apartamento does not get -n because it is governed by el

Also, the adjective agrees with the noun:

• la najbara apartamento = both are singular, and neither has -n
• if it were accusative for some other reason, both would change:
  • la najbaran apartamenton

But that is not what is happening here.

Here, el means out of or from inside.

So el la najbara apartamento means that the noise is coming out of the neighboring apartment.

That is a little more specific than just English from, because el suggests movement or origin from the inside of something.

Compare:

• el la domo = out of the house
• el la skatolo = out of the box
• bruo el la najbara apartamento = noise coming from the neighboring apartment

Because el is the better choice when something comes out of a place.

Esperanto has several words that English often translates as from, and they are not always interchangeable.

• el = out of, from inside
• de = often from, but also of, by, off, depending on context

In this sentence, the noise is understood as coming out of the apartment, so el feels natural.

If you used de, people might still understand you, but el is more precise and idiomatic here.

Because Esperanto has no indefinite article.

English distinguishes:

• a noise
• the noise

Esperanto only has la for the. If something is indefinite, you simply use the noun by itself.

So:

• bruo = a noise / noise
• la bruo = the noise

That is why bruon can mean a noise without needing a separate word for a.

Because -as is the Esperanto verb ending for the present tense.

The verb root here is aŭd- = hear. Then:

• aŭdas = hear / am hearing
• aŭdis = heard
• aŭdos = will hear
• aŭdi = to hear
• aŭdu = hear! / let ... hear

So Mi aŭdas means I hear or I am hearing, depending on context.

A common difficulty is the aŭ.

In Esperanto, aŭ is a diphthong, roughly like ow in English cow, though Esperanto pronunciation is more stable and regular.

So aŭdas is approximately:

• OW-dahs

A few pronunciation points:

• aŭ = like ow
• d is always a clear d
• a is like the a in father
• stress in Esperanto is usually on the second-to-last syllable

So:

• AŬ-das — stress on the first syllable, because there are only two syllables

bruon is usually pronounced in three sound units:

• BRU-o-n

Approximate English-style help:

• BROO-ohn

A few notes:

• bru- sounds like broo
• o is a clear pure vowel
• the final -n is pronounced

Also remember that Esperanto spelling is very regular, so you pronounce all the letters.

Najbaro means neighbor.
Najbara means neighboring / adjacent / next-door.

Esperanto often makes related words by changing endings:

• najbaro = a neighbor
• najbara = neighboring, neighborly

So:

• la najbara apartamento = the neighboring apartment

This is very common in Esperanto: the root stays the same, and the ending tells you the part of speech.

Because the speaker is referring to a specific apartment: the neighboring one.

In Esperanto, la is used much like English the. Even if something has not been mentioned before, you can still use la when it is identifiable from the situation.

For example:

• la suno = the sun
• la pordo = the door (when the relevant door is obvious)
• la najbara apartamento = the neighboring apartment (the specific apartment next door)

So the apartment is specific enough to take la.

Yes. Both are reasonable in English, depending on context.

Because Esperanto has no indefinite article, bruon can correspond to:

• a noise
• noise

The exact English wording depends on what sounds most natural in the situation.

So this sentence could mean:

• I hear a noise from the neighboring apartment
• I hear noise from the neighboring apartment

English chooses between those options more explicitly than Esperanto does.

Not exactly.

Bruo usually means noise, often an unpleasant, noticeable, or non-specific sound. It is not the most neutral word for sound in every context.

So:

• bruo = noise
• sono = sound (more general)

That means this sentence suggests the speaker hears some noise, not just any sound in a neutral scientific sense.

Yes, Esperanto word order is fairly flexible, because endings show the grammar clearly.

The neutral order here is:

• Mi aŭdas bruon el la najbara apartamento.

But other orders are possible for emphasis, such as:

• Bruon el la najbara apartamento mi aŭdas.
• El la najbara apartamento mi aŭdas bruon.

These all mean basically the same thing, though the emphasis changes.

Still, for learners, the original order is the most straightforward and natural one to use.

Because Esperanto pronouns are very regular.

Mi always means I. It does not change form the way English has I / me.

Examples:

• Mi aŭdas vin. = I hear you.
• Vi aŭdas min. = You hear me.

Instead of changing the pronoun form much, Esperanto often uses the -n ending to show object function. So the system is simpler and more regular than English.

In Esperanto, stress normally falls on the second-to-last syllable of each word.

So:

• Mi → only one syllable
• aŭdas → AŬ-das
• bruon → BRU-on
• el → one syllable
• la → one syllable
• najbara → naj-BA-ra
• apartamento → a-par-ta-MEN-to

That regular stress pattern is one of the things that makes Esperanto pronunciation easier to learn.