La najbarino, kies filino lernas en nia klaso, alportis ankaŭ kukon.

La is the definite article, meaning the. It shows that the speaker has a specific neighbor in mind: the neighbor.

Esperanto has only one article, la. It does not have separate words for a and an.

The ending -in- marks female sex. So:

• najbaro = neighbor
• najbarino = female neighbor

A learner will often see this pattern in words for people:

• filo = son
• filino = daughter
• patro = father
• patrino = mother

Kies means whose.

In this sentence, it connects the daughter to la najbarino:

• La najbarino ... alportis ankaŭ kukon = the neighbor also brought cake
• kies filino lernas en nia klaso = whose daughter studies in our class

So kies introduces a relative clause and shows possession at the same time.

No. Kies stays the same.

It does not change to match the person it refers to, and it does not take -j or -n. So you use kies whether the possessor is male, female, singular, or plural.

For example:

• la viro, kies filo... = the man whose son...
• la virino, kies filino... = the woman whose daughter...
• la homoj, kies infanoj... = the people whose children...

Because that part is a subordinate clause inserted into the middle of the sentence.

Esperanto punctuation often marks clause boundaries more regularly than English does. Since the kies-clause is inserted between the subject and the main verb, it is set off with commas on both sides:

• La najbarino, ... , alportis ankaŭ kukon.

So the commas help show the structure of the sentence.

Because lerni can work in different ways depending on the meaning.

• lerni ion = to learn something
• lerni en klaso / en lernejo = to study in a class / at a school

Here the meaning is about where the daughter studies, so Esperanto uses en:

• lernas en nia klaso = studies in our class

If you said lernas nian klason, it would sound like learns our class, which is not the intended idea.

Because kies already makes the noun specific: whose daughter.

In Esperanto, possessive words and similar determiners often make la unnecessary. So:

• kies filino = whose daughter
• not normally kies la filino

The same idea applies with words like mia, via, nia, and so on.

Because nia already means our, so it already determines the noun.

In Esperanto, you normally do not use la together with a possessive like mia, via, nia, ilia, etc. So:

• nia klaso = our class
• not usually la nia klaso

It can be broken down like this:

• al- = toward, to
• port- = carry
• -is = past tense

So alportis literally means something like carried to or brought.

Compared with portis (carried), alportis adds the idea of bringing something to a destination.

Because kukon is the direct object of alportis.

The neighbor is the one who did the action, and the cake is the thing brought. In Esperanto, the direct object usually gets -n:

• alportis kukon = brought a cake

Without -n, the grammar would be wrong here.

Because Esperanto has no indefinite article.

English says a cake, but Esperanto simply uses the noun by itself:

• kuko = cake / a cake
• kukon = cake / a cake as a direct object

The context tells you that the natural English translation is a cake.

Ankaŭ means also or too.

In this sentence, it most naturally means that the neighbor also brought a cake, probably in addition to something else already mentioned. Its position can shift depending on what the speaker wants to emphasize, but the version here is very natural:

• alportis ankaŭ kukon

So the idea is roughly: The neighbor ... also brought a cake.

Here it means class in the sense of a group of students.

That is a very common meaning of klaso in Esperanto. If you specifically wanted to say classroom as a physical room, you would usually use a different expression, such as klasĉambro.