Niaj geamikoj venos poste, se la trajno ne malfruos.

Ge- shows that the group includes both males and females. So geamikoj means a mixed-gender group of friends.

Breakdown:

• amiko = friend
• amikoj = friends
• geamikoj = male-and-female friends together

In modern usage, amikoj can often already mean friends in a general sense, but geamikoj makes the mixed group explicit.

Because it goes with the plural noun geamikoj.

In Esperanto, words that describe or determine a noun agree with it in number:

• nia amiko = our friend
• niaj amikoj = our friends

So niaj geamikoj is correct because geamikoj is plural.

Because neither word is a direct object here.

• Niaj geamikoj is the subject of venos
• la trajno is the subject of malfruos

The accusative ending -n is mainly used for direct objects and some expressions of motion/direction. Since both nouns are subjects, they stay without -n.

-Os is the future tense ending in Esperanto.

So:

• venas = comes / is coming
• venis = came
• venos = will come

Here venos shows that the friends’ coming is in the future.

Esperanto uses tense more directly according to the time of the action. Since the train’s lateness is also in the future, malfruos is natural.

So Esperanto can say:

• se la trajno ne malfruos = if the train is not late / if the train will not be late

English usually prefers present tense after if, but Esperanto does not have to copy that pattern.

It comes from:

• frua = early
• malfrua = late
• malfrui = to be late
• malfruos = will be late

The prefix mal- makes an opposite:

• bona = good → malbona = bad
• granda = big → malgranda = small
• frua = early → malfrua = late

With a train, English often says be delayed, but Esperanto literally says be late.

Poste is an adverb meaning later or afterwards.

In this sentence, it tells us when the friends will come:

• Niaj geamikoj venos poste = Our friends will come later

It is related to post = after.

Yes. Esperanto word order is fairly flexible, especially with adverbs.

These are all possible, with slightly different emphasis:

• Niaj geamikoj venos poste
• Poste niaj geamikoj venos
• Niaj geamikoj poste venos

The version in your sentence is a very normal, neutral order.

By itself, amiko means friend.

If you want boyfriend or girlfriend, Esperanto usually uses:

• koramiko = boyfriend
• koramikino = girlfriend

So geamikoj here clearly means friends, not romantic partners.

La is the definite article, meaning the.

It is used when the speaker and listener can identify which train is meant, or when the situation makes it clear. So la trajno means the train, not just any train.

If you said trajno without la, it would sound more like a train or some train, depending on context.