La aero ĉe la rivero kutime estas pura, sed hodiaŭ densa nebulo estis super la ponto.

Esperanto has no indefinite article. So where English says a dense fog, Esperanto simply says densa nebulo.

• nebulo = fog
• densa nebulo = a dense fog / dense fog

By contrast, Esperanto does have a definite article: la = the.

So:

• la ponto = the bridge
• densa nebulo = a dense fog / dense fog

La is the Esperanto definite article, meaning the.

It is used when the speaker means something specific or identifiable from the context:

• la aero = the air
• la rivero = the river
• la ponto = the bridge

In a sentence like this, those things are treated as known parts of the scene. Esperanto uses la much like English the, though exact usage can sometimes differ a little by context.

Also, la never changes:

• not for gender
• not for number
• not for case

So it is always just la.

Ĉe usually means at, by, near, or in the vicinity of.

So ĉe la rivero means something like:

• at the river
• by the river
• near the river

It does not necessarily mean literally in the river. For that, you would use en la rivero.

A useful comparison:

• ĉe la rivero = by/at the river
• en la rivero = in the river
• sur la rivero = on the river
• apud la rivero = next to the river

In this sentence, ĉe gives a natural idea of the air in the area around the river.

Kutime is an adverb meaning usually. It can move around fairly freely in Esperanto, as long as the meaning stays clear.

Here:

• La aero ĉe la rivero kutime estas pura

means The air by the river is usually clean/pure.

Placing kutime before estas is very natural, because it modifies the whole statement. But you could also hear:

• La aero ĉe la rivero estas kutime pura

That would also be correct.

Esperanto word order is often more flexible than English, though some placements sound more natural than others.

Pura is an adjective describing aero.

In Esperanto, adjectives usually end in -a:

• pura = pure / clean
• densa = dense / thick

Adjectives agree with the nouns they describe in number and case.

Here:

• aero is singular
• it is not accusative

So the adjective is just pura.

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

• puraj aeroj would be grammatically plural, though not very natural in meaning

If it were accusative, you would add -n to both noun and adjective where appropriate.

Yes. In Esperanto, adjectives can come before or after the noun, but before is the most common and neutral position.

So:

• densa nebulo = dense fog

is the normal order.

You might also see:

• nebulo densa

but that sounds more marked or poetic, or gives special emphasis.

For ordinary everyday Esperanto, adjective + noun is the safest pattern:

• pura aero
• densa nebulo
• granda ponto

Because there is no direct object here, and none of the words need the accusative.

The sentence is describing a situation:

• La aero ... estas pura
• densa nebulo estis super la ponto

These are statements with forms of esti (to be), not actions done to an object.

Also, prepositions like ĉe and super normally take their noun without -n:

• ĉe la rivero
• super la ponto

You may see -n after a preposition when it shows direction toward somewhere, but not here. This sentence describes location, not movement.

This is a very good question, because English learners often notice the tense contrast.

• estas = is / are
• estis = was / were

In the first clause:

• La aero ĉe la rivero kutime estas pura

the speaker is stating a general usual fact, so the present tense makes sense: is usually.

In the second clause:

• sed hodiaŭ densa nebulo estis super la ponto

the speaker is referring to a specific situation today, but from a viewpoint where it is treated as past: there was dense fog over the bridge.

This can mean, for example:

• earlier today there was fog
• today, at the time being talked about, there was fog

If the fog is still there right now, many speakers might instead say:

• sed hodiaŭ densa nebulo estas super la ponto

So the tense choice depends on the speaker’s time perspective.

Super means above or over.
Sur means on.

So:

• super la ponto = over the bridge / above the bridge
• sur la ponto = on the bridge

That is an important difference.

In this sentence, the fog is being described as hanging over the bridge, not sitting on top of it physically. So super is the correct choice.

Yes. Hodiaŭ means today, and like many adverbs in Esperanto, it can often move around.

The sentence has:

• sed hodiaŭ densa nebulo estis super la ponto

But these are also possible:

• sed densa nebulo hodiaŭ estis super la ponto
• sed densa nebulo estis hodiaŭ super la ponto

The first version, with hodiaŭ near the start of the clause, is very natural because it sets the time right away.

Esperanto allows some freedom, but word order is still used for style, rhythm, and emphasis.

Yes. Pura literally means pure, but depending on context it can also correspond to clean.

So La aero ... estas pura can naturally mean:

• the air is pure
• the air is clean

In many contexts about nature, weather, or pollution, pura aero is a normal way to talk about clean air.

So even if the literal translation suggests pure, the practical sense is often simply clean, fresh, unpolluted.

It has two main clauses joined by sed = but.

1. La aero ĉe la rivero kutime estas pura

   • subject: la aero
   • place phrase: ĉe la rivero
   • adverb: kutime
   • verb: estas
   • adjective complement: pura

2. sed hodiaŭ densa nebulo estis super la ponto

   • connector: sed
   • time word: hodiaŭ
   • subject: densa nebulo
   • verb: estis
   • place phrase: super la ponto

So the sentence contrasts:

• the usual condition by the river with
• the special condition today over the bridge.