Ich schließe die Balkontür, weil der Wind draußen stark ist.

In a normal German main clause, the finite verb usually comes in position 2.

So in Ich schließe die Balkontür:

• Ich = subject
• schließe = finite verb
• die Balkontür = object

This follows the standard German main-clause pattern:

Subject + verb + rest

English does something similar here: I close the balcony door.

Schließe is the correctly conjugated form of schließen for ich in the present tense.

The verb schließen changes like this:

• ich schließe = I close / I am closing
• du schließt
• er/sie/es schließt
• wir schließen
• ihr schließt
• sie/Sie schließen

So ich schließen would be ungrammatical, just like I to close in English.

It can mean either one, depending on context.

German often uses the simple present tense where English might use either:

• I close the balcony door
• I am closing the balcony door

So Ich schließe die Balkontür can describe:

• a habitual action, or
• something happening right now

In this sentence, it most naturally means I am closing the balcony door or I close the balcony door because of the strong wind outside.

Balkontür is a feminine noun, so its basic article is die.

• die Balkontür = the balcony door

Also, in this sentence it is the direct object of schließen, and feminine nouns keep die in both the nominative and accusative singular.

So:

• nominative: die Balkontür
• accusative: die Balkontür

That is why you still see die here.

It is in the accusative case because it is the thing being closed — the direct object of the verb schließen.

Ask yourself: What am I closing?
Answer: die Balkontür

That makes it the direct object, so German uses the accusative.

With a feminine singular noun, the article looks the same in nominative and accusative:

• nominative: die
• accusative: die

So the case is accusative, even though the form does not change.

Der Wind is the subject of the clause weil der Wind draußen stark ist.

That clause means because the wind outside is strong.

Here:

• der Wind = the wind
• draußen = outside
• stark = strong
• ist = is

Since Wind is a masculine noun, its nominative singular article is der.

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

So:

• main clause: Ich schließe die Balkontür
• subordinate clause: weil der Wind draußen stark ist

This verb-final structure is very common after words like:

• weil = because
• dass = that
• wenn = if/when
• obwohl = although

So weil der Wind draußen stark ist is correct, while weil der Wind draußen ist stark is not standard German.

In German, subordinate clauses are normally separated from the main clause with a comma.

Since weil der Wind draußen stark ist is a subordinate clause, the comma is required:

Ich schließe die Balkontür, weil der Wind draußen stark ist.

German uses commas more systematically here than English often does.

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

Compare:

• der starke Wind = the strong wind
  
  • here starke comes before the noun, so it takes an ending
• der Wind ist stark = the wind is strong
  
  • here stark comes after ist, so it does not take an ending

In your sentence, it is the second pattern:

der Wind ... stark ist

So stark stays unchanged.

Draußen means outside. It tells you where the wind is strong.

So:

• der Wind ist stark = the wind is strong
• der Wind draußen ist stark = the wind outside is strong

It adds location or setting.

In English, outside can sometimes sound like either an adverb or something almost adjectival. In German, draußen is commonly used as an adverb.

Yes, German word order is flexible, especially with adverbs, but the verb still has to stay at the end of the weil clause.

These are possible:

• weil der Wind draußen stark ist
• weil draußen der Wind stark ist

The original version sounds very natural.
Learners should keep in mind that the exact position of adverbs can shift, but the finite verb in a subordinate clause still goes at the end.

Yes. German very often combines nouns into one longer noun.

Here:

• Balkon = balcony
• Tür = door
• Balkontür = balcony door

The last part usually tells you the basic category of the thing:

• a Balkontür is a kind of Tür

This is very normal in German, and English sometimes writes similar ideas as two words instead.

Usually, schließen here means to close, not necessarily to lock.

So Ich schließe die Balkontür normally means:

• I close the balcony door
• I am closing the balcony door

If you specifically want to say lock, German often uses:

• abschließen = to lock

So:

• Ich schließe die Tür. = I close the door.
• Ich schließe die Tür ab. = I lock the door.

Yes, that would also be natural.

• weil der Wind draußen stark ist = because the wind outside is strong
• weil es draußen windig ist = because it is windy outside

The difference is mainly in expression:

• der Wind ... stark focuses on the wind
• es ... windig uses the adjective windy

Both are good German, but your original sentence is perfectly correct.

German does not use an auxiliary like English do in normal positive statements.

English can say:

• I close the door
• I do close the door for emphasis

German simply says:

• Ich schließe die Tür.

So the sentence is direct and straightforward, with no extra helper verb needed.