Breakdown of Когда ветер сильнее, листва шуршит громче.
Questions & Answers about 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.