Kiam la ŝipo alvenas en la havenon, multaj homoj rigardas ĝin de la ponto.

Because en can be followed by the accusative -n when it shows movement into a place.

• en la haveno = in the harbor
• en la havenon = into the harbor

So in la ŝipo alvenas en la havenon, the ship is arriving into the harbor, not simply being located there.

This is a very common Esperanto pattern:

• Mi estas en la domo = I am in the house
• Mi iras en la domon = I go into the house

Because ĝin is the accusative form of ĝi.

Here, ĝin refers back to la ŝipo and functions as the direct object of rigardas:

• ĝi = it
• ĝin = it, as a direct object

So:

• multaj homoj rigardas ĝin = many people are watching it

Esperanto marks direct objects with -n, including pronouns.

Kiam means when and introduces a time clause.

So:

• Kiam la ŝipo alvenas en la havenon = When the ship arrives in the harbor

This clause sets the time for the main action:

• multaj homoj rigardas ĝin de la ponto = many people watch it from the bridge

A native English speaker can think of kiam as the normal word for both:

• when? as a question word
• when... as a connector in a sentence

Esperanto uses the simple present -as for present actions, habitual actions, and often for general narrative statements. So alvenas can correspond to several English possibilities depending on context:

• arrives
• is arriving
• sometimes even does arrive

In this sentence, Kiam la ŝipo alvenas... most naturally means When the ship arrives... or When the ship is arriving...

Esperanto does not force the same tense distinctions that English often does. The context supplies the exact nuance.

Yes, very possibly.

Esperanto often allows both patterns with verbs of arrival or movement:

• alveni al la haveno = arrive at the harbor
• alveni en la havenon = arrive into the harbor

The sentence you have is slightly more vivid in showing movement into the harbor. English would usually just say arrive at the harbor, but Esperanto can express the idea of entering the place more directly.

So both are possible, but en la havenon emphasizes motion into the harbor area.

Vidi means to see. It often refers to simply perceiving something with your eyes.

Rigardi means to look at or to watch. It suggests directed attention.

So:

• Mi vidas la ŝipon = I see the ship
• Mi rigardas la ŝipon = I am looking at / watching the ship

In this sentence, the people are not just noticing the ship; they are actively watching it. That is why rigardas is the better choice.

La is the definite article, meaning the.

It appears here because the speaker is talking about specific things:

• la ŝipo = the ship
• la havenon = the harbor
• la ponto = the bridge

Esperanto has only one article, la, and it does not change for gender, number, or case. Even when a noun gets -n, the article stays the same:

• la haveno
• la havenon

De la ponto literally means from the bridge.

It tells you the viewpoint or source from which the people are watching:

• they are watching the ship from the bridge

By contrast:

• sur la ponto = on the bridge

So the difference is:

• de la ponto emphasizes the place from which the looking happens
• sur la ponto emphasizes physical position on top of the bridge

In many situations, both could make sense, but de la ponto is very natural if the bridge is being treated as the observation point.

Normally ponto means bridge in the usual sense, such as a bridge over water or a roadway.

If the meaning were specifically the ship's bridge, the context would usually need to make that clear. In this sentence, the most natural reading is that people are watching the ship from a bridge near the harbor.

So a learner should probably understand:

• de la ponto = from the bridge near the harbor

not:

• from the ship's command bridge

Yes. Esperanto word order is fairly flexible because grammar is shown clearly by endings, especially the accusative -n.

This sentence uses a very natural order:

1. time clause first: Kiam la ŝipo alvenas en la havenon
2. main clause next: multaj homoj rigardas ĝin de la ponto

But other orders are possible if you want a different emphasis. For example:

• Multaj homoj rigardas ĝin de la ponto, kiam la ŝipo alvenas en la havenon.

That still means the same basic thing.

Even with flexible word order, the original version is probably the clearest and most natural for a learner.

Multaj homoj means many people.

• homo = person
• homoj = people
• multa = much / many
• multaj = many, plural to agree with a plural noun

In Esperanto, adjectives agree with nouns in number, and also in case if needed. Since homoj is plural, multaj must also be plural.

So:

• multa homo would be incorrect here
• multaj homoj is correct

Because the sentence begins with a subordinate clause introduced by kiam.

So the structure is:

• Kiam la ŝipo alvenas en la havenon, = subordinate time clause
• multaj homoj rigardas ĝin de la ponto. = main clause

Using a comma here is normal and helps make the sentence easier to read. It works much like English punctuation in a sentence such as:

• When the ship arrives in the harbor, many people watch it from the bridge.