La poŝtkarto de la morgaŭ alvenonta gasto estis tre amuza.

Alvenonta is a future active participle.

It comes from:

• alveni = to arrive
• -ont- = about to / going to / will
• -a = adjective ending

So:

• alvenonta gasto = a guest who will arrive / an arriving-soon guest
• with morgaŭ: la morgaŭ alvenonta gasto = the guest who will arrive tomorrow

This is a compact way to say what could also be expressed with a relative clause, such as la gasto, kiu alvenos morgaŭ.

Because morgaŭ modifies the idea of arriving.

So morgaŭ alvenonta means arriving tomorrow.

In Esperanto, adverbs like morgaŭ can come before the word or phrase they modify. Here it naturally goes before the participle phrase:

• la morgaŭ alvenonta gasto

This is similar to saying in English the tomorrow-arriving guest, though English normally prefers the guest arriving tomorrow.

It describes gasto.

You can tell because:

• alvenonta has the adjective ending -a
• adjectives describe nouns
• the noun right after it is gasto

So:

• la morgaŭ alvenonta gasto = the guest who will arrive tomorrow

It does not describe poŝtkarto.

Because there are two separate noun phrases, and both are definite:

1. La poŝtkarto = the postcard
2. la morgaŭ alvenonta gasto = the guest who will arrive tomorrow

Esperanto uses la wherever definiteness is needed. It does not avoid repeating the the way English sometimes does in certain structures.

So the sentence is built like this:

• La poŝtkarto
• de la morgaŭ alvenonta gasto
• estis tre amuza

De usually means of, from, or sometimes marks possession, origin, or relation.

Here it links poŝtkarto and gasto:

• la poŝtkarto de la gasto

Depending on context, that can mean:

• the guest's postcard
• the postcard from the guest
• the postcard of the guest

In real usage, Esperanto de often covers relationships that English expresses in several different ways.

Because it is the subject of the sentence, not the direct object.

In Esperanto:

• the direct object gets -n
• the subject does not

Here the verb is estis = was, and the thing that was very amusing is la poŝtkarto.

So:

• La poŝtkarto ... estis tre amuza.

No accusative ending is needed.

Because gasto is inside a de-phrase:

• de la morgaŭ alvenonta gasto

After a preposition like de, you normally do not use -n just for being an object-like idea. The preposition already shows the relationship.

So:

• de la gasto = of/from the guest
• not de la gaston

Because amuza is a predicate adjective after estis.

In Esperanto, after verbs like esti (to be), the adjective agrees with the noun in number and accusative status. Since poŝtkarto is singular and not accusative, the adjective is also singular and not accusative:

• poŝtkarto
• estis tre amuza

If the noun were plural, you would say:

• La poŝtkartoj estis tre amuzaj.

Estis is the past tense of esti (to be).

So it means was.

That tells you the whole statement is in the past:

• La poŝtkarto ... estis tre amuza.
• The postcard ... was very amusing.

Even though alvenonta refers to a future arrival, that future is relative to the guest, not to the main verb tense. In other words, the guest was still going to arrive at the time being talked about.

Because alvenonta lets Esperanto turn the verb idea into an adjective modifying gasto.

Compare:

• la morgaŭ alvenonta gasto = the guest arriving tomorrow
• la gasto, kiu alvenos morgaŭ = the guest who will arrive tomorrow

Both are correct. The participle version is shorter and more compact.

An English speaker may find the relative clause version easier at first, but participles are very common and useful in Esperanto.

Yes. A very natural alternative is:

• La poŝtkarto de la gasto, kiu alvenos morgaŭ, estis tre amuza.

That means essentially the same thing as the original.

The original sentence uses a participial adjective:

• la morgaŭ alvenonta gasto

The rephrased version uses:

• la gasto, kiu alvenos morgaŭ

Learners often find the second version easier to understand at first.

Not completely. Esperanto word order is fairly flexible, though some orders sound more natural than others.

For example, this is possible:

• La poŝtkarto de la gasto morgaŭ alvenonta estis tre amuza.

But the original:

• La poŝtkarto de la morgaŭ alvenonta gasto estis tre amuza.

is clearer and more elegant, because morgaŭ alvenonta stays together directly before gasto.

So while Esperanto allows flexibility, good style usually keeps related words close together.

Yes, more or less. Esperanto allows this kind of compact modifier more naturally than English does.

A literal breakdown is something like:

• morgaŭ = tomorrow
• alvenonta = going to arrive
• gasto = guest

So literally:

• the tomorrow-going-to-arrive guest

But in normal English, you would usually say:

• the guest arriving tomorrow
• or the guest who will arrive tomorrow

So the Esperanto is normal, even if a word-for-word English version sounds strange.

Tre is an adverb meaning very.

It modifies amuza:

• tre amuza = very amusing

This is straightforward, but it is a useful pattern to notice:

• tre bona = very good
• tre interesa = very interesting
• tre amuza = very amusing

In Esperanto, tre often works just like English very before adjectives and adverbs.