kono yama ha aki no kouyou ga totemo kirei desu.

They are marking different things.

• この山は = as for this mountain / speaking of this mountain
  Here, は marks the topic.
• 秋の紅葉が = the autumn leaves
  Here, が marks what is being described as beautiful.

So the structure is:

• Topic: this mountain
• What is beautiful about it: its autumn foliage

A natural way to understand it is:

• As for this mountain, its autumn leaves are very beautiful.

This kind of A は B が adjective pattern is very common in Japanese.

この山 means this mountain.

• この = this
• 山 = mountain

With は, it becomes the topic:

• この山は = as for this mountain

It does not necessarily mean the mountain itself is beautiful. It sets up the mountain as the thing we are talking about, and then the rest of the sentence explains what about it is beautiful.

紅葉 already means autumn leaves / autumn foliage, so 秋の紅葉 can feel a little more explicit, like:

• the autumn foliage in autumn
• or more naturally, the fall colors

In real Japanese, 紅葉 by itself is often enough. But 秋の紅葉 is still understandable and emphasizes the seasonal aspect.

Here, の links 秋 and 紅葉, giving the sense of:

• autumn’s foliage
• or the foliage in autumn

の is a linking particle. It often shows possession, but it has a broader use than English 's.

In 秋の紅葉, it connects autumn and autumn leaves/foliage. A natural English interpretation is:

• the autumn foliage
• the leaves in autumn

So here の is not really strict possession. It is more like the foliage associated with autumn.

Because 紅葉 is the thing being described by きれいです.

In other words, what is very beautiful?

• 秋の紅葉

So:

• 秋の紅葉がとてもきれいです = the autumn leaves are very beautiful

This is a common pattern with adjectives in Japanese:

• X が adjective です

For example:

• 花がきれいです = The flowers are beautiful
• 景色がすばらしいです = The scenery is wonderful

Because きれい only uses な when it comes directly before a noun.

• きれいな山 = a beautiful mountain

But when きれい is the main predicate of the sentence, you do not use な:

• 山がきれいです = The mountain is beautiful

So in your sentence:

• 秋の紅葉がとてもきれいです = correct
• 秋の紅葉がとてもきれいなです = incorrect

In Japanese grammar, きれい is usually treated as a na-adjective.

That means:

• before a noun: きれいな
• at the end of a sentence: きれいです

Examples:

• きれいな花 = a beautiful flower
• 花がきれいです = the flower is beautiful

So even though it behaves differently from い-adjectives like 高い, it still works as an adjective in meaning.

とても means very.

It modifies きれいです:

• とてもきれいです = is very beautiful

Without it, the sentence would simply be:

• この山は秋の紅葉がきれいです = This mountain’s autumn leaves are beautiful

Adding とても just strengthens the description.

Not exactly.

The sentence is more specifically about the mountain’s autumn foliage, not the mountain in general.

A closer interpretation is:

• This mountain has very beautiful autumn leaves.
• As for this mountain, the autumn foliage is very beautiful.

If you wanted to say This mountain is very beautiful in autumn, you would usually phrase it differently.

It can.

For example:

• この山の秋の紅葉はとてもきれいです。

This means something like:

• This mountain’s autumn foliage is very beautiful.

That version focuses more directly on the autumn foliage of this mountain as the topic.

Your original sentence:

• この山は秋の紅葉がとてもきれいです。

has a slightly different structure:

• As for this mountain, its autumn foliage is very beautiful.

Both are natural, but the original sentence uses the very common topic + subject pattern.

Yes, but the nuance changes.

• この山は秋の紅葉がとてもきれいです。
  Focus: on this mountain, the autumn leaves are beautiful.

• この山は秋の紅葉はとてもきれいです。
  This sounds more contrastive, like:
  As for the autumn leaves, they are very beautiful
  perhaps implying that something else is not.

So が is the more neutral and natural choice here.

In this sentence, it is most naturally read as こうよう.

• 紅葉（こうよう） often means autumn foliage / fall colors in a broad scenic sense.
• 紅葉（もみじ） often refers to maple leaves or the leaves themselves more concretely.

Since the sentence is talking about a mountain’s seasonal beauty, こうよう is the most likely reading.

Japanese often expresses ideas differently from English.

English might say:

• This mountain has beautiful autumn leaves.

Japanese instead says something closer to:

• As for this mountain, the autumn leaves are beautiful.

So rather than using a verb meaning have, Japanese often uses a topic-comment structure to describe a feature of something.

A useful way to see it is:

A は B が adjective です

In this sentence:

• A = この山
• B = 秋の紅葉
• adjective = きれい

So:

• この山は秋の紅葉がとてもきれいです。

This pattern is often used when talking about one aspect or feature of something:

• この店はラーメンがおいしいです。
  This shop’s ramen is delicious.
• 彼は目が大きいです。
  He has big eyes.

So your sentence follows a very common and useful structure.