La nokta aero ĉe la maro estas pli trankvila ol en la urbo.

Nokta means night, nighttime, or of the night as an adjective.

It comes from the root nokt- plus the adjective ending -a:

• nokto = night
• nokta = night-time / nightly / of the night

So la nokta aero means the night air.

La is the definite article, like the in English.

So:

• la nokta aero = the night air
• nokta aero = night air or a night air

In this sentence, la makes the phrase sound like a specific, understood thing: the night air by the sea. That is very natural in Esperanto, just as in English.

Because it is the subject of the sentence, not a direct object.

In Esperanto, the -n ending is mainly used for:

• the direct object
• motion toward somewhere in some cases

Here:

• La nokta aero = the thing being described
• estas pli trankvila = is calmer

So aero stays in the basic nominative form, without -n.

Because ĉe means at, by, or near, while en means in.

So:

• ĉe la maro = by the sea / near the sea
• en la maro = in the sea

The sentence is talking about the air near the sea, not air physically inside the sea. So ĉe la maro is the correct choice.

Because trankvila is an adjective describing the noun aero.

• trankvila = calm, მშვიდ? no, English only: calm
• trankvile = calmly

After estas, Esperanto normally uses an adjective when describing the subject:

• La aero estas trankvila. = The air is calm.

If you said trankvile, that would be an adverb, and it would not fit this sentence.

Because it agrees with aero, which is singular.

In Esperanto, adjectives agree with the nouns they describe in number and case:

• singular noun → -a
• plural noun → -aj

So:

• aero = air, singular
• trankvila = calm, singular adjective

If the noun were plural, the adjective would be plural too:

• La noktaj ventoj estas pli trankvilaj... = The night winds are calmer...

This is the normal Esperanto pattern for comparisons.

• pli = more
• ol = than

So:

• pli trankvila = more calm = calmer
• pli trankvila ol ... = calmer than ...

This works just like English more ... than ..., except Esperanto uses the same pattern very consistently.

Examples:

• Ŝi estas pli alta ol mi. = She is taller than me.
• La akvo estas pli malvarma ol hieraŭ. = The water is colder than yesterday.

Because Esperanto often leaves out words that are clearly understood from context.

The full idea is:

• La nokta aero ĉe la maro estas pli trankvila ol ĝi estas en la urbo.

But that sounds unnecessarily heavy. So Esperanto, like English, usually shortens it:

• ... pli trankvila ol en la urbo.
• literally: ... calmer than in the city.

The missing idea is understood as something like than it is in the city.

Because the comparison is really between two situations or locations:

• the night air by the sea
• the night air in the city

So en la urbo means in the city, and the sentence implies:

• The night air by the sea is calmer than the night air is in the city.

Using en is important because it shows location inside the city.

Yes. Esperanto word order is fairly flexible, because endings show the grammatical roles more clearly than in English.

For example, these are all possible:

• La nokta aero ĉe la maro estas pli trankvila ol en la urbo.
• Ĉe la maro la nokta aero estas pli trankvila ol en la urbo.

Both mean essentially the same thing. The difference is mainly one of emphasis or style.

The original version begins with La nokta aero, so it focuses first on the night air.

Yes, aero is a normal singular noun meaning air. Even though English often treats air as an uncountable noun, Esperanto still uses the regular noun form:

• aero = air
• la aero = the air

So the singular is perfectly normal here. The sentence is talking about the air as one general thing in that place and time.

Not really. In this sentence, nokta simply means of the night or nighttime.

So:

• la nokta aero = the night air

English nocturnal usually describes animals or behavior that is active at night, so it would sound odd here. Nokta is much broader and more straightforward: it just connects something with the night.

Yes, that is close to the structure of the Esperanto sentence, but the most natural English is:

• The night air by the sea is calmer than in the city.

A few notes:

• ĉe la maro is better translated as by the sea or near the sea, not usually at the sea
• pli trankvila is most naturally calmer
• ol en la urbo means than in the city, with the rest understood

So the Esperanto structure is fairly transparent, but smooth English usually makes a few small adjustments.