Ich ziehe den Vorhang zu, weil die Sonne so hell ist.

Because the verb here is zuziehen, which is a separable verb.

• Infinitive: den Vorhang zuziehen
• In a main clause, the conjugated part goes to position 2:
  • Ich ziehe den Vorhang zu.

So:

• ziehe = the conjugated verb part
• zu = the separable prefix, which moves to the end of the clause

This is very common in German:

• Ich mache die Tür zu.
• Er steht früh auf.

Because Vorhang is the direct object of the verb, so it is in the accusative case.

Vorhang is a masculine noun:

• nominative: der Vorhang
• accusative: den Vorhang

In this sentence:

• Ich = subject
• ziehe = verb
• den Vorhang = direct object

So German uses den, not der.

A useful comparison:

• Der Vorhang ist teuer.
  Here der Vorhang is the subject.
• Ich ziehe den Vorhang zu.
  Here den Vorhang is the object.

It means to draw the curtain closed or to pull the curtain shut.

The verb zuziehen often suggests closing something by pulling it. That makes it especially natural with things like:

• curtains
• doors
• zippers
• blinds

So for a curtain, zuziehen is more idiomatic than a more general verb like schließen in many everyday contexts.

Because weil introduces a subordinate clause, and in German subordinate clauses, the conjugated verb usually goes to the end.

Compare:

• Main clause: Die Sonne ist so hell.
• After weil: weil die Sonne so hell ist

That verb-final pattern is one of the most important features of German word order.

Other examples:

• weil ich müde bin
• weil er keine Zeit hat
• weil wir nach Hause gehen

Because weil die Sonne so hell ist is a subordinate clause, and German normally separates subordinate clauses from main clauses with a comma.

So:

• Ich ziehe den Vorhang zu, weil die Sonne so hell ist.

This is standard German punctuation.

Both can work, but they are not exactly the same in feeling.

• sehr hell = very bright
• so hell = so bright

In a sentence with weil, so often sounds very natural because it emphasizes the degree in a way that fits the explanation:

• ..., weil die Sonne so hell ist. = ..., because the sun is so bright.

You could also say:

• ..., weil die Sonne sehr hell ist.

That is also grammatical, but so hell often feels a bit more expressive in this kind of sentence.

Yes. You can also say:

Weil die Sonne so hell ist, ziehe ich den Vorhang zu.

This means the same thing.

But notice what happens in the main clause after the subordinate clause comes first:

• not ..., ich ziehe ...
• but ..., ziehe ich ...

That is because German main clauses still keep the verb in the second position, and the whole subordinate clause counts as the first element.

So:

• Ich ziehe den Vorhang zu, weil ...
• Weil ..., ziehe ich den Vorhang zu.

Both are correct.

Because Sonne is a feminine noun in German.

Its article is:

• die Sonne

This has to be memorized along with the noun, since grammatical gender in German does not always match what an English speaker expects.

Also, remember that all nouns are capitalized in German, so Sonne and Vorhang both begin with capital letters.

It is the present tense, and in German the present tense is often used for both:

• something happening right now
• a general present action
• sometimes even a near-future action, depending on context

So Ich ziehe den Vorhang zu can mean:

• I am closing the curtain
• I close the curtain

In normal everyday German, the simple present often does the job that English sometimes gives to the present progressive.

In some contexts, you may hear similar everyday expressions, but for curtains, den Vorhang zuziehen is especially natural because a curtain is typically drawn or pulled shut.

So:

• den Vorhang zuziehen = very idiomatic
• schließen or other alternatives may be possible in some contexts, but they are less specific

In other words, zuziehen matches the physical action particularly well here.