Aŭtune la vento estas pli malvarma, sed la arboj en la parko ankoraŭ estas belaj.

Aŭtune is an adverb meaning in autumn or during autumn. Esperanto often turns time words into adverbs with -e:

• printempo = spring
• printempe = in spring
• somero = summer
• somere = in summer
• aŭtuno = autumn
• aŭtune = in autumn
• vintro = winter
• vintre = in winter

So Aŭtune la vento estas pli malvarma means In autumn, the wind is colder.

You could also say en aŭtuno, but aŭtune is very common and elegant.

La is the definite article, meaning the.

In Esperanto, just like in English, you use la when talking about something specific or something understood in context. Here la vento can mean the wind in a general, familiar sense, like the wind we experience in autumn.

So this is natural:

• la vento = the wind
• la arboj = the trees
• la parko = the park

Esperanto has only one article, la. There is no separate word for a or an.

Because malvarma is an adjective describing la vento.

In the sentence:

• la vento estas pli malvarma

the word after estas describes what the wind is like, so it must be an adjective, not an adverb.

• malvarma = cold
• pli malvarma = colder

An adverb like malvarme would describe how something happens, not what something is like.

Compare:

• La vento estas malvarma = The wind is cold
• La vento blovas malvarme would be unnatural; adverbs are used differently

So pli malvarma is correct because it agrees with vento.

Pli means more.

So:

• pli malvarma = more cold = colder
• pli bela = more beautiful
• pli granda = bigger

Esperanto usually makes comparisons with pli instead of changing the adjective itself.

Examples:

• bela = beautiful
• pli bela = more beautiful
• malvarma = cold
• pli malvarma = colder

If you want the most, you use plej:

• plej bela = most beautiful

The ending -j marks the plural in Esperanto.

• arbo = tree
• arboj = trees

Adjectives must agree with the nouns they describe, so:

• bela arbo = a beautiful tree
• belaj arboj = beautiful trees

In your sentence:

• la arboj ... estas belaj

both arboj and belaj are plural, so both take -j.

Because there is no direct object here.

The -n ending is usually used for:

1. the direct object
2. motion toward something in some cases

This sentence is mostly about description and location:

• la vento estas pli malvarma
• la arboj en la parko ankoraŭ estas belaj

No noun is receiving the action of a verb, so no direct object appears.

Also, en la parko means in the park as a location, not movement into the park. So no -n is needed there either.

Compare:

• La arboj estas en la parko = The trees are in the park
• Mi iras en la parkon = I go into the park

Ankoraŭ means still.

So:

• la arboj en la parko ankoraŭ estas belaj = the trees in the park are still beautiful

Its position is flexible, but here it naturally modifies the whole idea that the trees remain beautiful.

You may also see:

• la arboj ankoraŭ estas belaj
• ankoraŭ la arboj estas belaj is less natural in most contexts

The version in the sentence is very normal and clear.

En means in.

So:

• en la parko = in the park

This phrase tells you where the trees are. It is a prepositional phrase modifying la arboj.

Breaking it down:

• en = in
• la = the
• parko = park

So la arboj en la parko means the trees in the park.

Yes. Sed means but and connects two contrasting ideas.

Here the contrast is:

• in autumn, the wind is colder
• but the trees in the park are still beautiful

So sed functions very much like English but.

Aŭtune is pronounced approximately like ow-TOO-neh.

A few helpful points:

• aŭ sounds like the ow in cow
• stress in Esperanto usually falls on the second-to-last syllable
• so Aŭ-TU-ne is stressed on TU

That gives:

• Aŭtune → ow-TOO-neh

These words contain -oj, a very common Esperanto ending.

• arboj is approximately AR-boy
• belaj is approximately BEH-lie or BEH-lahy, depending on how you hear the Esperanto aj sound

More exactly:

• oj sounds roughly like English oy
• aj sounds roughly like the y sound in my

So:

• arboj = AR-boy
• belaj = BEH-lie with the stress on BEH

Remember that Esperanto stress is usually on the second-to-last syllable:

• AR-boj
• BE-laj

Yes. In English, both in autumn and in the autumn are possible, though in autumn is often more natural in general statements.

So Aŭtune can match either of these, depending on style:

• In autumn, the wind is colder...
• In the autumn, the wind is colder...

The Esperanto word itself does not force one version over the other.

Yes, that is completely normal in Esperanto.

English adjectives do not change for singular or plural:

• beautiful tree
• beautiful trees

But Esperanto adjectives do change to match the noun:

• bela arbo = beautiful tree
• belaj arboj = beautiful trees

So:

• la arboj ... estas belaj = the trees ... are beautiful

This agreement happens whether the adjective comes before the noun or after estas.