Die weiße Gardine am Fenster bewegt sich im Wind.

Because Gardine is feminine singular, and it is in the nominative case here because it is the subject of the sentence.

After die in the feminine nominative singular, the adjective takes -e:

• die weiße Gardine
• die kleine Lampe
• die schöne Blume

So weiße is the correct adjective ending.

Because Gardine is a feminine noun in German: die Gardine.

German nouns have grammatical gender, and the article must match that gender:

• der = masculine
• die = feminine
• das = neuter

So the sentence starts with Die weiße Gardine because Gardine is feminine.

Not quite. Both can be translated as curtain, but they often suggest different things.

• Gardine usually means a light, thin curtain, often the kind hanging in front of a window.
• Vorhang often means a heavier curtain or drape.

So in this sentence, Gardine gives the image of a light curtain moving in the wind.

Am is a contraction of an dem:

• an dem Fenster → am Fenster

This is very common in German.

Here, am Fenster means something like at the window or by the window.

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

• accusative for movement toward a place
• dative for location

Here, the curtain is simply located at/by the window, not moving toward it, so German uses the dative:

• am Fenster = an dem Fenster

Compare:

• Die Gardine hängt am Fenster. = location
• Er geht ans Fenster. = movement toward the window

Because the verb here is sich bewegen, which is commonly used reflexively and means to move.

So:

• Die Gardine bewegt sich. = The curtain moves / is moving.

The word sich is the reflexive pronoun, like oneself in form, though in English we usually do not say moves itself here.

In a normal German main clause, the finite verb usually comes in second position.

Here the structure is:

• Die weiße Gardine = position 1
• bewegt = position 2
• sich = comes after the finite verb

So:

• Die weiße Gardine bewegt sich im Wind.

This is standard word order for a reflexive verb in a main clause.

Im is a contraction of in dem:

• in dem Wind → im Wind

So im Wind literally means in the wind.

In English we usually say in the wind or sometimes just in the breeze, depending on context.

Again, in is a two-way preposition:

• accusative for movement into something
• dative for location or situation

Here, im Wind describes the circumstances in which the curtain is moving. It is not moving into the wind; it is moving in the wind. So German uses the dative.

Compare:

• Die Gardine bewegt sich im Wind. = in the wind
• Der Vogel fliegt in den Wind. = into the wind

The letter ß is called Eszett or scharfes S. It is pronounced like a normal s sound, not like z.

So weiße sounds roughly like VY-seh.

A few helpful points:

• w in German sounds like English v
• ei sounds like English eye
• ß sounds like s

So weiß sounds roughly like vice in English, and weiße adds a final -uh sound.

Yes. Weiß is the base adjective meaning white.

When an adjective comes before a noun in German, it usually takes an ending:

• weiß = white
• die weiße Gardine = the white curtain

So weiße is just the inflected form of weiß.

Most naturally, it describes the curtain: the white curtain at the window.

So the sentence is understood as:

• The white curtain at the window is moving in the wind.

Grammatically, German often allows this kind of phrase without making the relationship overly explicit, and context tells you what is most natural.

Yes. German word order is flexible, as long as the finite verb stays in second position in a main clause.

For example:

• Im Wind bewegt sich die weiße Gardine am Fenster.

This emphasizes im Wind more strongly.

But the original sentence is the most neutral and straightforward version:

• Die weiße Gardine am Fenster bewegt sich im Wind.