Wenn der Wind stark ist, bewegt sich der Rollladen am Fenster.

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

• Wenn der Wind stark ist
• literally: When/if the wind strong is

That is normal German word order after wenn.

German uses a comma to separate a subordinate clause from a main clause.

So in this sentence:

• Wenn der Wind stark ist, = subordinate clause
• bewegt sich der Rollladen am Fenster. = main clause

This comma is required in standard German.

Both are possible in German, but the word order depends on what comes first.

German main clauses follow the V2 rule: the conjugated verb must be in the second position.

Here, the whole wenn-clause takes the first position:

• Wenn der Wind stark ist, = position 1
• bewegt = position 2

After that comes the subject:

• der Rollladen

So:

• Wenn der Wind stark ist, bewegt sich der Rollladen am Fenster.

If there were no opening wenn-clause, you would normally say:

• Der Rollladen bewegt sich am Fenster.

Because in a main clause, the conjugated verb must come in the second position. The reflexive pronoun sich does not take that position instead of the verb.

So the order is:

• bewegt = conjugated verb
• sich = reflexive pronoun

That gives:

• bewegt sich der Rollladen

In a subordinate clause, however, you often do see sich before the infinitive or past participle, for example:

• ..., weil sich der Rollladen bewegt.

Yes. Here sich bewegen means to move or to shift/move by itself.

Compare:

• Ich bewege den Tisch. = I move the table.
  (bewegen used transitively: moving something)

• Der Tisch bewegt sich. = The table moves / is moving.
  (sich bewegen: the thing itself moves)

So in your sentence, the shutter is not being moved by a person; it is moving on its own because of the wind.

It can often mean either when or if, depending on context.

In this sentence, the most natural sense is something like:

• when/whenever the wind is strong

because it sounds like a general situation or repeated event.

If the sentence were about a specific possible condition, English might prefer if. German still often uses wenn for that.

So wenn is very commonly used for:

• when in repeated situations
• if in conditions

Because stark is used here as a predicate adjective, after the verb sein.

In German, adjectives used after verbs like sein, werden, and bleiben are not inflected.

So:

• Der Wind ist stark.
• not Der Wind ist starker.

You only add adjective endings when the adjective comes before a noun:

• der starke Wind = the strong wind

Der Wind is in the nominative case because it is the subject of the first clause.

• der Wind = the thing doing the being in ist stark

You can identify it by asking: who or what is strong?

• der Wind

Der Rollladen is also nominative, because it is the subject of the main clause.

Who or what is moving?

• der Rollladen

Even though it comes after the verb, it is still the subject.

Am Fenster is a contraction of:

• an dem Fenster

Here, an means something like at, on, or by, depending on context.

So der Rollladen am Fenster means:

• the roller shutter at the window
• or more naturally, the roller shutter on the window / by the window

It identifies which shutter is meant.

Because this sentence describes location, not movement toward a destination.

German two-way prepositions like an use:

• dative for location: am Fenster (an dem Fenster)
• accusative for direction toward something: ans Fenster (an das Fenster)

Here the shutter is already located at the window, so German uses dative:

• am Fenster

If something moved toward the window, you might use ans Fenster instead.

A Rollladen is a roller shutter, the kind that can be pulled or rolled down over a window, often on the outside of a building.

It is not exactly the same as:

• Jalousie = blinds
• Vorhang = curtain

So Rollladen specifically refers to a shutter that rolls up and down.

Because all German nouns are capitalized.

So in this sentence, these words are capitalized because they are nouns:

• Wind
• Rollladen
• Fenster

This is one of the most noticeable spelling features of German.

Rollladen is a masculine noun, so its dictionary form is:

• der Rollladen

Unfortunately, grammatical gender in German often has to be learned together with the noun. It is best to memorize nouns with their article:

• der Wind
• der Rollladen
• das Fenster

That way, you also learn the forms you will need later for case changes.

Yes. You could say:

• Der Rollladen am Fenster bewegt sich, wenn der Wind stark ist.

This means the same thing. The difference is mainly emphasis and sentence flow.

Starting with Wenn der Wind stark ist puts the condition first. Starting with Der Rollladen am Fenster puts the subject first. Both are natural German.