Vespere multaj homoj promenas sur la ponto kaj rigardas la akvon.

Vespere is an adverb, meaning in the evening or during the evening. The ending -e marks adverbs in Esperanto.

• vespero = evening
• vespere = in the evening / evening-time

Esperanto often uses time words as adverbs this way:

• matene = in the morning
• tage = by day / during the day
• nokte = at night

So Vespere multaj homoj... means In the evening, many people...

Multaj means many and it works like an adjective here, so it matches the noun homoj in number.

• homo = person
• homoj = people
• multa homo = much person, which does not make sense here
• multaj homoj = many people

Because homoj is plural, multaj also takes -j.

The ending -j marks the plural in Esperanto.

• homo = person
• homoj = people

Since the sentence is talking about more than one person, the plural is required.

Promenas is the present tense form of promeni.

• promeni = to walk / to stroll
• promenas = walk / are walking

The ending -as always marks the present tense in Esperanto. It can describe:

• something happening now
• something habitual or generally true
• something vividly described in the present

Here it means that the people walk or are walking on the bridge.

Because this phrase shows location, not motion toward a place.

• sur la ponto = on the bridge
• sur la ponton = onto the bridge

In Esperanto, -n is often used after a preposition when there is movement toward a destination. But here the people are already on the bridge while walking there, so the normal form is sur la ponto.

The -n marks the direct object.

In this sentence, the people are doing two actions:

• promenas = they walk
• rigardas la akvon = they look at the water

What are they looking at? La akvon. That makes la akvon the direct object, so it takes -n.

• akvo = water
• la akvon = the water, as the direct object

Because they have different jobs in the sentence.

• sur la ponto is part of a prepositional phrase showing location
• la akvon is the direct object of rigardas

So:

• ponto is not a direct object here
• akvon is a direct object, so it gets -n

This is a very common pattern in Esperanto.

La is the definite article, meaning the.

Esperanto uses la when the speaker has a specific thing in mind or presents it as identifiable in the situation.

So here:

• la ponto = the bridge
• la akvon = the water

Even if English might sometimes say water without the, Esperanto may still use la when it refers to the visible water in that scene, such as the water under the bridge.

Also, la never changes form:

• singular or plural: still la
• nominative or accusative: still la

Because the same subject applies to both verbs.

The subject is multaj homoj, and it belongs to both:

• multaj homoj promenas
• multaj homoj rigardas la akvon

Esperanto, like English, does not need to repeat the subject if it stays the same. So kaj rigardas la akvon means and look at the water with the same people doing both actions.

Yes, Esperanto word order is fairly flexible.

The neutral order here is:

Vespere multaj homoj promenas sur la ponto kaj rigardas la akvon.

But other orders are also possible, for example:

• Multaj homoj promenas sur la ponto vespere kaj rigardas la akvon.
• Sur la ponto multaj homoj promenas vespere kaj rigardas la akvon.

The endings help show what each word is doing, especially -n for direct objects, so Esperanto can allow more freedom than English. Still, the original order sounds natural and straightforward.

It is very close, but the grammar works a little differently.

In English, you usually say look at something.
In Esperanto, rigardi normally takes a direct object:

• rigardi la akvon = to look at the water

So Esperanto does not need a separate word for at here. That is why la akvon gets the direct object ending -n.

Sometimes, but vespere is the more natural and compact choice here.

• vespere = in the evening
• en la vespero = in the evening, but more literal and usually less idiomatic in a simple sentence like this

Esperanto often prefers adverbial time forms such as:

• matene
• tagmeze
• vespere
• nokte

So vespere is exactly what a learner should expect in this kind of sentence.