Wenn der Wind stark ist, rascheln die Blätter im Garten.

Because wenn introduces a subordinate clause. In German, the conjugated verb usually goes to the end of a subordinate clause.

So:

• der Wind ist stark = the wind is strong
• wenn der Wind stark ist = when the wind is strong

This is one of the most important German word-order patterns to learn:

• main clause: the conjugated verb is usually in position 2
• subordinate clause: the conjugated verb usually goes to the end

Because the sentence starts with a subordinate clause, and after that German uses normal main clause word order, where the conjugated verb must come in position 1 or 2 depending on what comes first—but more specifically, the first slot is already taken by the whole Wenn... clause.

So the structure is:

• Wenn der Wind stark ist, = first element
• rascheln = verb in second position
• die Blätter im Garten = the rest

This is called verb-second word order in the main clause.

Compare:

• Die Blätter rascheln im Garten.
• Wenn der Wind stark ist, rascheln die Blätter im Garten.

In both cases, the main clause verb is in the second position.

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

• der Wind = subject of ist
• die Blätter = subject of rascheln

The articles show this:

• der Wind = masculine singular nominative
• die Blätter = plural nominative

In this sentence:

• Wind is masculine: der Wind
• Blätter is plural of das Blatt, and the plural article is die

Because stark is being used predicatively, not directly before a noun.

Compare:

• der Wind ist stark = the wind is strong
  → stark comes after ist, so it does not take an adjective ending

• der starke Wind = the strong wind
  → starke comes before the noun, so it does take an ending

This is the same idea as in English:

• the wind is strong
• the strong wind

Rascheln usually means to rustle or to make a rustling sound.

It is often used for sounds made by things like:

• leaves
• paper
• clothing
• dry grass

So die Blätter rascheln is a very natural way to say the leaves rustle.

It is a sound-based verb, a bit like knistern for crackling or rauschen for rushing/noise.

Because im is a contraction of in dem.

• in dem Garten → im Garten

This is very common in German.

Here, Garten is masculine (der Garten), and after in in this sentence it is in the dative case, so:

• in dem Garten
• contracted: im Garten

Because in is a two-way preposition. It can take:

• accusative when there is movement toward somewhere
• dative when there is location and no movement

Here, im Garten means in the garden as a location. The leaves are rustling there; they are not moving into the garden.

So:

• im Garten = location → dative
• in den Garten = into the garden → accusative

Sometimes, but not with exactly the same feel.

• wenn can mean when or sometimes if, depending on context
• falls means if, often with a slightly more conditional or less certain tone

In this sentence, wenn is very natural because it describes a general situation:

• When the wind is strong, the leaves rustle in the garden.

If you say falls, it sounds more like:

• If the wind happens to be strong, the leaves rustle in the garden.

So wenn is the better choice for a general fact or repeated situation.

Yes. A very common alternative is:

Die Blätter rascheln im Garten, wenn der Wind stark ist.

This means the same thing. The difference is mainly what comes first and what gets emphasis.

• Starting with Wenn der Wind stark ist... highlights the condition first.
• Starting with Die Blätter rascheln... highlights the main action first.

Both are correct.

A rough guide:

• Blätter sounds roughly like BLET-ter, with the ä like the vowel in bet but shorter and tenser depending on accent.
• rascheln sounds roughly like RAH-sheln or RASH-eln, depending on how simplified the English guide is.

A few pronunciation notes:

• ä in Blätter is not the same as English a in late
• sch is like English sh
• the r in German can vary by accent
• the final -eln in rascheln is often spoken fairly lightly

If you want a very rough pronunciation of the whole sentence:

Venn dair Vint shtark ist, RAH-sheln dee BLET-ter im GAR-ten.

That is only approximate, but it may help at first.

The singular is das Blatt.

So:

• das Blatt = the leaf / the sheet
• die Blätter = the leaves / sheets

Notice the plural changes both:

• the article: das → die
• the vowel: a → ä
• and adds -er

This is a common plural pattern in German.

It most naturally sounds like a general truth or a habitual situation:

When the wind is strong, the leaves rustle in the garden.

German often uses the present tense for this kind of general statement.

Depending on context, it could also refer to a particular situation, but without more context, it sounds like a general observation about what happens under those conditions.