Mia onklo, kiu loĝas ĉe la maro, venos al nia urbo dimanĉe.

Mia and nia are possessive adjectives.

• mia = my
• nia = our

They work like regular adjectives and describe a noun:

• mia onklo = my uncle
• nia urbo = our town

In Esperanto, possessive words like mia, via, lia, ŝia, ĝia, nia, and ilia are very common and straightforward.

In Esperanto, a possessive adjective usually makes la unnecessary.

So you normally say:

• mia onklo, not la mia onklo
• nia urbo, not la nia urbo

That is similar to English: we say my uncle, not the my uncle.

Forms like la mia... can exist in special emphatic or literary contexts, but for normal Esperanto, just use the possessive adjective by itself.

Here kiu is a relative pronoun, meaning who.

It refers back to onklo:

• Mia onklo, kiu loĝas ĉe la maro...
• My uncle, who lives by the sea...

So kiu introduces extra information about onklo.

A useful point: in Esperanto, kiu can mean who, which, or that, depending on the context.

Because kiu is the subject of loĝas.

In the clause:

• kiu loĝas ĉe la maro

the uncle is the one doing the living, so kiu is the subject. Subjects do not take -n.

You would use kiun only if it were a direct object.

For example:

• La onklo, kiun mi vidis... = The uncle whom I saw...

Here, in your sentence, the correct form is kiu.

The commas show that this is a non-restrictive relative clause: it adds extra information, but it does not identify which uncle is meant.

So the sentence is saying something like:

• My uncle — who lives by the sea — will come...

The core sentence still works without that middle part:

• Mia onklo venos al nia urbo dimanĉe.

That is why commas are used.

If the clause were necessary to identify which person you meant, punctuation could work differently, but in this sentence it is clearly extra information.

These are verb tense endings:

• -as = present tense
• -os = future tense

So:

• loĝas = lives / is living
• venos = will come

Esperanto verb endings are very regular:

• -as present
• -is past
• -os future
• -us conditional
• -u command / wish
• -i infinitive

That regularity is one of the easiest parts of Esperanto grammar.

Because veni means to come, while iri means to go.

Esperanto uses this distinction much like English does:

• veni = movement toward the speaker or the reference point
• iri = movement away from the speaker or toward some other place

In this sentence, the destination is nia urbo (our town), which is treated as the relevant place, so venos is natural:

• He will come to our town

If you said iros, it would sound more like he will go to our town, which would usually be said from a different point of view.

Ĉe means something like at, by, or near.

So:

• loĝas ĉe la maro = lives by the sea / near the sea

That is the natural choice here.

By contrast:

• en la maro = in the sea

That would suggest being physically in the water, which is obviously not what is meant.

So ĉe la maro is the normal way to say someone lives by the sea or on the coast.

Because la maro means the sea.

In Esperanto, la is often used when something is understood as a specific or familiar thing, and the sea is commonly treated that way, just as in English.

So:

• ĉe la maro = by the sea

Without la, maro would sound more like a sea or sea as an idea, which is not what this sentence wants.

Al means to and shows direction toward a destination.

So:

• venos al nia urbo = will come to our town

This is the normal preposition for movement toward a place.

Also, because al already shows direction, you do not add -n to urbo here. Esperanto usually does not use both a directional preposition and an accusative ending for the same job.

Dimanĉe is an adverbial time expression, meaning on Sunday.

Esperanto often uses -e forms to express when something happens:

• dimanĉe = on Sunday
• matene = in the morning
• vespere = in the evening

So here:

• venos ... dimanĉe = will come ... on Sunday

This is a very natural Esperanto pattern.

You may also encounter time expressions with -n in Esperanto, but dimanĉe is a standard and common way to say on Sunday.

Yes, Esperanto word order is fairly flexible, because grammar is shown clearly by endings and prepositions.

For example, these are all possible:

• Mia onklo, kiu loĝas ĉe la maro, venos al nia urbo dimanĉe.
• Dimanĉe mia onklo, kiu loĝas ĉe la maro, venos al nia urbo.
• Al nia urbo mia onklo, kiu loĝas ĉe la maro, venos dimanĉe.

However, some orders sound more natural than others depending on what you want to emphasize.

The original sentence is neutral and very natural: first the person, then the extra description, then the action, then the destination, then the time.