La venontan monaton mia onklino sendos poŝtkartojn al ĉiuj geavoj, kiuj restos hejme.

Here -n is being used for time, not for a direct object.

Esperanto often uses the accusative ending -n for expressions of time, measure, or direction. So:

• La venontan monaton = next month
• literally something like the coming month used as a time expression

This is often called the accusative of time. It answers when?

So in this sentence:

• La venontan monaton mia onklino sendos...
• = Next month my aunt will send...

Venontan comes from veni = to come.

More specifically:

• veni = to come
• venonta = coming / about to come / upcoming
• venontan = accusative singular form of venonta

So la venontan monaton means the coming month, which in natural English is usually next month.

Because it refers to a specific month: the upcoming one.

In Esperanto, la is often used when something is clearly identifiable or specific. La venontan monaton points to one particular month, not just any month.

So:

• monato = a month
• la venonta monato = the coming month / the next month

In the sentence, this phrase is then put into the accusative of time:

• la venontan monaton

Because possessives like mia, via, lia, ŝia, etc. usually make la unnecessary.

So:

• la onklino = the aunt
• mia onklino = my aunt

You normally do not say la mia onklino.

Sendos is the future tense of sendi = to send.

Breakdown:

• send- = send
• -os = future tense

So:

• sendas = sends / is sending
• sendis = sent
• sendos = will send

Therefore:

• mia onklino sendos = my aunt will send

Because it is the direct object of sendos.

Your aunt is sending what? → poŝtkartojn = postcards

Breakdown:

• poŝtkarto = postcard
• poŝtkartoj = postcards
• poŝtkartojn = postcards as a direct object

The endings show:

• -j = plural
• -n = accusative

So poŝtkartojn means postcards as the thing being sent.

Because geavoj is not the direct object here. It is the recipient of the postcards, and Esperanto usually marks that with al.

So:

• sendi ion = to send something
• sendi ion al iu = to send something to someone

In this sentence:

• poŝtkartojn = the thing sent
• al ĉiuj geavoj = to all grandparents

Since geavoj follows the preposition al, it does not take -n for direct object.

Also:

• ĉiuj agrees with geavoj
• both are plural

Geavoj means grandparents.

The prefix ge- shows a mixed-sex group or both sexes together. So:

• avo = grandfather
• avino = grandmother
• geavoj = grandparents

This is very common with family words in the plural:

• gepatroj = parents
• gefratoj = siblings / brothers and sisters
• geavoj = grandparents

Because kiuj is plural.

The relative pronoun must match the noun it refers to. Here it refers to geavoj, which is plural:

• geavoj = grandparents
• kiuj = who / which (plural)

So:

• la geavo, kiu restos hejme = the grandparent who will stay home
• la geavoj, kiuj restos hejme = the grandparents who will stay home

It is a relative clause describing which grandparents are meant.

So the sentence is not talking about all grandparents everywhere. It is talking about:

• all the grandparents who will stay at home

The clause:

• kiuj restos hejme
• = who will stay home

It narrows down the group of grandparents being referred to.

Not necessarily.

In Esperanto, commas are commonly used before subordinate clauses, including relative clauses, more regularly than in English. So the comma here does not automatically create the same meaning contrast that English punctuation often does.

In English, compare:

• the grandparents who will stay home = restrictive
• the grandparents, who will stay home = often non-restrictive

In Esperanto, the comma before kiuj is normal and does not by itself force that distinction in the same way. You understand the meaning mainly from context.

Grammatically, a plural kiuj could in theory match either plural noun, but in this sentence it is understood to refer to geavoj.

Why?

Because restos hejme = will stay at home makes sense for people, not for postcards. So the meaning points clearly to the grandparents.

If someone wanted to make it even clearer, they could rephrase, but in normal usage this sentence is not really confusing.

Because hejme is the normal Esperanto way to say at home.

It is an adverbial form:

• hejmo = home
• hejme = at home

So:

• resti hejme = to stay at home

This is more natural than resti en hejmo, which would sound more literal, like stay in a home/house.

Yes. Esperanto word order is fairly flexible because endings show the grammatical roles.

For example, you could also say:

• Mia onklino sendos poŝtkartojn al ĉiuj geavoj, kiuj restos hejme, la venontan monaton.

That would still mean basically the same thing.

The original order puts the time expression first:

• La venontan monaton...
• = Next month...

This gives the time a little more emphasis, much like English does when you start a sentence with Next month.