Post kelkaj minutoj, la trajno alvenos.

Because -os is the Esperanto future-tense verb ending.

• alveni = to arrive
• alvenas = arrives / is arriving
• alvenis = arrived
• alvenos = will arrive

So la trajno alvenos means the train will arrive.

Post means after.

So post kelkaj minutoj literally means after a few minutes.

In English, we often say in a few minutes, but Esperanto commonly uses post in this kind of sentence. So even if the English translation says in a few minutes, the Esperanto wording with post is perfectly natural.

Because this phrase is the object of the preposition post, not a direct object of a verb.

In Esperanto, nouns after a preposition usually do not take the accusative -n.

So:

• post kelkaj minutoj = after a few minutes

Also, kelkaj must agree with minutoj, so both are plural:

• kelkaj = a few / some
• minutoj = minutes

That is why both words have -j.

Because adjectives and adjective-like words in Esperanto agree with the nouns they describe in number and case.

Here:

• minutoj is plural
• so kelkaj must also be plural

Compare:

• kelka minuto = some minute / a certain minute
• kelkaj minutoj = a few minutes

So the -j on both words shows plural agreement.

La is the definite article, meaning the.

So la trajno means the train, not just a train.

Esperanto uses la when the thing is specific or already known from context. In a sentence like this, the speaker usually has a particular train in mind, so la trajno is natural.

Veni means to come, while alveni means to arrive.

The prefix al- often adds the idea of movement to a destination.

So:

• veni = come
• alveni = arrive

In this sentence, alvenos is better because the train is reaching its destination or stopping point. English also usually says the train will arrive, not the train will come, in this context.

The word order is natural, but Esperanto is fairly flexible.

This sentence is:

• Post kelkaj minutoj, la trajno alvenos.

A very neutral English-like structure is:

• time expression first
• then subject
• then verb

You could also say:

• La trajno alvenos post kelkaj minutoj.

That means the same thing. The version with Post kelkaj minutoj first gives a little more emphasis to the time.

The comma separates the introductory time phrase from the main clause:

• Post kelkaj minutoj, = after a few minutes
• la trajno alvenos. = the train will arrive

This kind of comma is common and helps readability. Esperanto punctuation is not always identical to English punctuation, but this comma is completely normal.

Yes.

Kelkaj often means:

• some
• a few
• several (depending on context)

In this sentence, English usually prefers a few minutes, so that is the most natural translation. But the core idea is simply an indefinite small number of minutes.

A simple pronunciation guide:

• kelkaj ≈ KELL-kai
• alvenos ≈ ahl-VEH-nos

A few useful Esperanto pronunciation points here:

• j sounds like English y
• aj sounds roughly like eye
• stress is always on the second-to-last syllable

So:

• kel-kaj → stress on kel
• al-ve-nos → stress on ve

Yes, but it is a slightly different structure.

• kelkaj minutoj = a few minutes
• kelke da minutoj = a quantity of minutes / a few minutes

Both are grammatical and natural. For a beginner, kelkaj minutoj is simpler and very common.

The sentence with kelke da would be:

• Post kelke da minutoj, la trajno alvenos.

That still means After a few minutes, the train will arrive.

Not literally.

Literally, post kelkaj minutoj means after a few minutes.

But in actual use, it often corresponds to English in a few minutes, because English and Esperanto package time expressions differently here. So the literal wording and the natural translation are not always identical, even though the meaning is the same.