Ĉe la strando ni aŭdas la ondojn en la vespero.

Because it has two endings at once:

• -j = plural
• -n = direct object

The basic word is ondo = wave.
So:

• ondo = a wave
• ondoj = waves
• ondon = a wave, as a direct object
• ondojn = waves, as a direct object

In this sentence, the waves are the thing that we hear, so they take the accusative -n.

Here en la vespero is a prepositional phrase, and the preposition en already shows the relationship: in / during the evening.

Normally, nouns after prepositions do not take -n unless there is a special reason, such as showing movement toward something in certain expressions.

So:

• en la vespero = in the evening
• la vesperon would be a direct object, which is not the role here

Ĉe usually means something like at, by, near, or in the area of.

So ĉe la strando means at the beach or by the beach.

It does not always mean physically on top of something. It often gives a general location.

Both can be possible, but they are slightly different in nuance.

• ĉe la strando = at the beach / by the beach
• sur la strando = on the beach

So ĉe is a bit broader and less physically exact. It places you in that location generally.
If you want to stress that you are literally standing or sitting on the sand, sur la strando may feel more precise.

Esperanto uses la as the definite article, like the in English.

In this sentence:

• la strando = the beach
• la vespero = the evening

Each noun phrase gets its own article if it is definite. Esperanto does not usually drop la just because it already appeared earlier in the sentence.

Aŭdas is the present tense form of the verb aŭdi = to hear.

Esperanto verb endings are very regular:

• -i = infinitive → aŭdi = to hear
• -as = present → aŭdas = hear / are hearing
• -is = past
• -os = future
• -us = conditional
• -u = command / volitive

So ni aŭdas means we hear or we are hearing.

Because Esperanto verbs show tense, but not person or number.

So aŭdas can mean:

• I hear
• you hear
• he hears
• she hears
• it hears
• we hear
• they hear

The subject pronoun tells you who is doing the action.
That is why ni is needed: ni aŭdas = we hear.

Yes, Esperanto word order is fairly flexible, because endings show the grammatical roles.

This sentence could be rearranged in different ways, for example:

• Ni aŭdas la ondojn ĉe la strando en la vespero.
• En la vespero ni aŭdas la ondojn ĉe la strando.

The basic meaning stays similar.
Still, some orders sound more natural depending on what you want to emphasize. The original sentence sounds normal and clear.

A rough guide:

• ĉ sounds like ch in church
• aŭ sounds like ow in cow
• j sounds like English y
• ŝ sounds like sh
• ŭ is a short glide, similar to a w-like sound in a diphthong

So:

• Ĉe sounds roughly like cheh
• aŭdas sounds roughly like OW-das

Also, Esperanto stress is almost always on the second-to-last syllable:

• STRAN-do
• aŭ-DAS
• on-DOJN
• ve-SPE-ro

They are very close in meaning.

• en la vespero = in the evening
• vespere = in the evening / evenings, adverbially

The form vespere is often a little more compact and idiomatic when talking about time.
So you could also say:

Ĉe la strando ni aŭdas la ondojn vespere.

That would still be natural.

Because -o is the normal noun ending in Esperanto.

So:

• strando = beach
• ondo = wave
• vespero = evening

This is one of the most important patterns in Esperanto:

• -o = noun
• -a = adjective
• -e = adverb
• -i = infinitive verb

These regular endings make it easier to recognize what each word is doing in the sentence.

It can often be understood more generally as in the evening, depending on context.

Esperanto la is used in some time expressions where English may sound more general. So en la vespero can still be a natural way to say in the evening without strongly emphasizing one unique evening.

If the context is about a particular evening, it can also mean on that evening / in that evening period. Context decides how specific it feels.