У клёна листья уже жёлтые, а у дуба они ещё зелёные.

Here у + Genitive means something like by / on / in the case of a person or thing, and in natural English it often corresponds to the ... of ... or a ... has ....

So:

• У клёна листья уже жёлтые = The maple’s leaves are already yellow
• literally, something like At the maple, the leaves are already yellow
• У дуба они ещё зелёные = The oak’s leaves are still green

This у + Genitive pattern is very common in Russian when talking about something belonging to or associated with someone/something:

• У меня есть книга = I have a book
• У дома крыша красная = The house has a red roof

So in this sentence, у is not really about physical location in the usual sense. It marks the tree that the leaves belong to.

Because after у, Russian uses the Genitive case.

Base forms:

• клён = maple
• дуб = oak

Genitive singular:

• клёна
• дуба

So:

• у клёна
• у дуба

This is exactly the normal case pattern after у.

Листья is plural: leaves.

The singular is:

• лист = leaf

The plural nominative is:

• листья = leaves

This is an irregular-looking plural, but it is very common in Russian. So:

• лист жёлтый = the leaf is yellow
• листья жёлтые = the leaves are yellow

In the sentence, листья is the subject of the first clause.

In present-tense Russian, the verb to be is usually omitted.

So where English says:

• The leaves are yellow
• They are still green

Russian normally just says:

• Листья жёлтые
• Они ещё зелёные

That is completely normal. Russian does have forms of to be, but in the present tense they are usually not used in simple statements like this.

Yes, они could be omitted in many contexts:

• У клёна листья уже жёлтые, а у дуба ещё зелёные.

That would still sound natural.

But они is added here to make the contrast clearer and to explicitly refer back to листья. It is a bit like saying:

• ...and the oak’s ones are still green
• or ...and they, on the oak, are still green

So они is not strictly necessary, but it helps the sentence feel balanced and clear.

Because they agree with листья, which is plural nominative.

Russian adjectives change to match the noun in:

• gender
• number
• case

Here:

• листья = plural
• subject of the clause = nominative
• so the adjectives are plural nominative too

That gives:

• жёлтые
• зелёные

Compare:

• лист жёлтый = singular masculine
• листья жёлтые = plural

They create the contrast in time.

• уже = already
• ещё = still / yet, depending on context

So:

• уже жёлтые = already yellow
• ещё зелёные = still green

The idea is that the maple has changed color, but the oak has not changed yet.

This pairing is very common in Russian:

• Он уже дома. = He is already home.
• Она ещё на работе. = She is still at work.

А often marks a contrast or comparison, while и usually means simple and.

Here the sentence contrasts two trees:

• the maple: leaves already yellow
• the oak: leaves still green

So а is the natural choice:

• ..., а у дуба... = ..., whereas for the oak... / but the oak’s...

It is not a strong contradiction like English but always is, but it does set up a contrast.

Yes, Russian word order is flexible, but this version is very natural.

Current structure:

• У клёна листья уже жёлтые, а у дуба они ещё зелёные.

This puts the trees first, so the sentence is organized as a comparison between the maple and the oak.

You could also hear variations such as:

• Листья у клёна уже жёлтые, а у дуба ещё зелёные.
• У дуба листья ещё зелёные, а у клёна уже жёлтые.

The meaning stays similar, but the focus shifts slightly depending on what is placed first.

Yes, but that would mean something a bit different.

• кленовые листья = maple leaves
• дубовые листья is less natural for actual leaves because дубовый very often means made of oak / oak-wooden rather than simply belonging to an oak tree

In this sentence, у клёна and у дуба are better because the point is not naming a type of leaf in general. It is talking about the leaves on this tree.

So у + Genitive is the most natural pattern here.

Yes, the standard spelling is жёлтые with ё.

In many printed texts, Russian often writes е instead of ё, so you may see:

• желтые
• зеленые

But the correct pronunciation is still:

• жёлтые
• зелёные

For learners, writing ё is helpful because it shows both the pronunciation and the stress more clearly.

That is close to the structure, and it can help you understand the grammar, but it is not good natural English.

A more natural English translation is:

• The maple’s leaves are already yellow, while the oak’s are still green.

So the literal structure can help with learning:

• у клёна = at/by the maple

But the real meaning in context is possessive/associative:

• the maple’s leaves

Yes, very common.

Russian often uses у + Genitive when talking about a feature, part, or characteristic of something:

• У машины сломано колесо. = The car has a broken wheel.
• У дома большие окна. = The house has big windows.
• У берёзы белая кора. = The birch has white bark.

So У клёна листья... fits a very common and natural Russian pattern.