Когда ветер сильнее, листва шуршит громче.

Because Когда ветер сильнее is a subordinate clause introduced by когда. In Russian, a subordinate clause is normally separated from the main clause with a comma.

So the structure is:

Когда ветер сильнее, листва шуршит громче.
= When/whenever the wind is stronger, the foliage rustles louder.

If you reverse the order, you still keep the comma:

Листва шуршит громче, когда ветер сильнее.

It can be understood as either when or whenever, depending on context.

In a sentence like this, which sounds like a general truth, English often prefers whenever:

Whenever the wind is stronger, the foliage rustles louder.

But Russian still uses когда. It does not need a separate word for whenever in many cases.

They are both comparative forms.

• сильный → сильнее = stronger
• громкий / громко → громче = louder

Russian often uses these short comparative forms where English uses more + adjective/adverb or -er forms.

So:

• ветер сильнее = the wind is stronger
• шуршит громче = rustles louder

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

So:

• ветер сильнее literally looks like wind stronger
• but it means the wind is stronger

This is completely normal in Russian. The same thing happens in many simple present-tense sentences:

• Он дома = He is at home
• Она счастлива = She is happy

Here it works like the comparative part of the predicate: ветер сильнее = the wind is stronger.

This can be confusing because Russian comparative forms like сильнее often look the same whether they relate to an adjective or an adverb.

In this sentence:

• сильнее describes the condition/state of ветер
• громче describes how шуршит happens

So in English terms, you can think of:

• сильнее = predicate adjective comparative
• громче = adverb comparative

But in Russian, the same comparative form is often used in both roles.

Листва means foliage or leafage as a collective mass. It refers to leaves as a whole.

Листья means leaves as individual countable leaves.

So:

• листва шуршит = the foliage rustles
• листья шуршат = the leaves rustle

Both are possible, but they feel slightly different:

• листва sounds more collective, natural, descriptive, sometimes more literary
• листья focuses more on the individual leaves

Also notice the verb agreement:

• листва шуршит — singular
• листья шуршат — plural

Both are in the nominative case because they are the subjects of their clauses.

• ветер = nominative singular
• листва = nominative singular

In the sentence:

• ветер is the subject of the subordinate clause
• листва is the subject of the main clause

Шуршать means to rustle, to make a soft dry brushing sound, like leaves, paper, grass, clothing, and so on.

So листва шуршит is a very natural phrase for the sound foliage makes in the wind.

It has a sound-symbolic quality: шур- imitates a rustling noise.

Yes. Russian word order is flexible.

For example:

Листва шуршит громче, когда ветер сильнее.

This means the same thing: The foliage rustles louder when the wind is stronger.

The original version puts the when-clause first, which is very natural if you want to set up the condition first.

You usually would not say that here.

Russian normally prefers the simple comparative forms:

• сильнее
• громче

Using более is possible in some contexts, especially in more formal or analytical language, but here it would sound less natural.

So the natural phrasing is:

• ветер сильнее
• шуршит громче

not usually:

• ветер более сильный
• шуршит более громко

It actually is based on громко, but the comparative form is irregular-looking:

• громко = loudly
• громче = louder

Similarly:

• тихо = quietly
• тише = more quietly / quieter

Russian comparatives often do not look completely predictable from the basic form, so they are worth memorizing individually.

Most naturally, it sounds like a general observation or general rule:

When/whenever the wind is stronger, the foliage rustles louder.

Without extra context, it does not strongly point to one specific moment. Russian often uses the present tense this way for general truths and repeated situations.

A very literal structure is:

When the wind is stronger, the foliage rustles louder.

A more natural English paraphrase might be:

• The stronger the wind, the louder the foliage rustles.
• Whenever the wind picks up, the foliage rustles more loudly.

But the Russian sentence itself is simple and natural as it stands.