Mi aŭdas la najbarinon frapi ĉe la pordo.

Because la najbarinon is the direct object of aŭdas.

• Mi aŭdas ... = I hear ...
• The thing or person being heard takes the -n ending in Esperanto.

So:

• la najbarino = the female neighbor
• la najbarinon = the female neighbor as the object of the verb

This is one of the most important uses of -n in Esperanto.

After verbs of perception like aŭdi (to hear) and vidi (to see), Esperanto often uses an infinitive to describe what someone is perceived doing.

So:

• Mi aŭdas la najbarinon frapi = I hear the neighbor knock / I hear the neighbor knocking

Here, frapi is not the main verb of the sentence. The main verb is aŭdas.
The infinitive frapi tells you what I hear her do.

This is similar to English patterns like:

• I hear her sing
• I saw him run

It means I hear the neighbor knocking.

In this structure:

• la najbarinon is the person perceived
• frapi is the action she is doing

So the sense is:

• I hear [the neighbor] [knock]

It does not mean that someone is hitting her. If you wanted that kind of meaning, the sentence structure would need to be different.

Because frapi is an infinitive verb, and infinitives do not take -n.

The -n ending goes on nouns, adjectives, and certain adverbial expressions, but not on infinitive verbs.

So:

• najbarinon gets -n because it is a noun functioning as the object
• frapi stays frapi because it is an infinitive

Ĉe usually means at, by, or near.

So ĉe la pordo means:

• at the door
• by the door

In this sentence, it tells us where the knocking is happening.

Esperanto uses ĉe when something happens at a location. That is very natural here, because knocking happens at the door, not necessarily into it or toward it.

Yes, but the meaning shifts slightly.

• frapi la pordon = to hit/knock the door
• frapi ĉe la pordo = to knock at the door

Both can be understood, but frapi ĉe la pordo is often preferred when you mean the normal action of knocking in order to be let in.

Using la pordon makes the door the direct object of frapi.
Using ĉe la pordo focuses more on the location of the knocking.

Because it is not the direct object here.

The direct object of the main verb aŭdas is la najbarinon.
And in the phrase ĉe la pordo, la pordo is the object of the preposition ĉe.

In Esperanto, nouns after prepositions normally do not take -n unless there is some special reason, such as direction in certain expressions.

So:

• ĉe la pordo = correct
• not ĉe la pordon in this sentence

Grammatically, it says I hear the neighbor knock.

Esperanto builds this as:

• I hear the neighbor
  • doing the action of knocking

In real usage, of course, what you literally hear is the sound, but the grammar focuses on the person being perceived performing the action.

This is normal and matches how many languages express perception.

Yes, but the infinitive structure is the simplest and most common here.

• Mi aŭdas la najbarinon frapi is very natural.

A participial version may be possible in some contexts, but for a learner, the infinitive after aŭdi is the pattern to remember.

So if you want to say:

• I hear her sing
• I hear them argue
• I hear the neighbor knock

the infinitive construction is an excellent standard choice.

Yes, Esperanto word order is fairly flexible, but this order is the most straightforward.

The sentence is arranged like this:

• Mi = subject
• aŭdas = main verb
• la najbarinon = object of the main verb
• frapi = action I hear her doing
• ĉe la pordo = place

That order is easy to understand and sounds natural.

Because Esperanto marks the direct object with -n, you can move things around more than in English. But for learners, the standard order is usually best unless you have a reason to emphasize something.

Yes. Najbarino specifically means female neighbor.

• najbaro = neighbor (traditionally masculine or unspecified depending on context, but often understood as male in traditional usage)
• najbarino = female neighbor

If you do not want to specify female, modern Esperanto speakers may choose different strategies depending on style and community preference. But in this sentence, najbarinon clearly tells you the neighbor is female.

Yes. The infinitive itself does not force a single knock.

Frapi can refer to the action in a general way, and context decides whether it means:

• one knock
• repeated knocking
• the act of knocking in progress

So Mi aŭdas la najbarinon frapi ĉe la pordo can naturally be understood as I hear the neighbor knocking at the door.

Yes. This is a very common Esperanto pattern with perception verbs.

For example:

• Mi vidas la knabon kuri. = I see the boy run/running.
• Mi sentas la teron tremi. = I feel the ground shake/shaking.
• Mi aŭdas la najbarinon frapi. = I hear the neighbor knock/knocking.

So a useful pattern to learn is:

• [subject] + [perception verb] + [object with -n] + [infinitive]

That pattern will help you understand and build many similar sentences.