Wenn die neue Glühbirne funktioniert, öffne ich die Jalousie und lasse den Rollladen oben.

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

So:

• Wenn die neue Glühbirne funktioniert
• literally follows the pattern When/if the new light bulb works

This is one of the most important word-order rules in German.

German main clauses follow the verb-second rule. The finite verb must come in the second position.

Here, the whole wenn clause counts as the first element of the sentence:

• Wenn die neue Glühbirne funktioniert, = position 1
• öffne = position 2
• ich comes after that

So öffne ich is correct here. If the sentence started directly with ich, then it would be ich öffne.

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

For English speakers, this can feel confusing, because English often separates the two more clearly. In German:

• wenn can mean when for repeated or expected situations
• wenn can also mean if, especially in everyday German

If you want a more clearly conditional word, German also has falls. But wenn is very normal here.

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

Since Wenn die neue Glühbirne funktioniert is a subordinate clause, it must be set off with a comma before the main clause begins.

This comma is not optional in standard German.

German often uses the present tense to talk about the future when the meaning is clear from context.

So:

• Wenn die neue Glühbirne funktioniert, öffne ich...

can refer to something that will happen later, even though the verbs are present tense.

This works a lot like English in sentences such as:

• If it works, I open... is unusual in English
• but If it works, I’ll open... is normal

German very often keeps simple present where English prefers will.

This is about both gender and case.

• die neue Glühbirne is the subject of funktioniert, so it is in the nominative
• die Jalousie is the direct object of öffne, so it is in the accusative
• den Rollladen is the direct object of lasse, so it is also in the accusative

The reason the articles look different is gender:

• die Glühbirne = feminine
• die Jalousie = feminine
• der Rollladen = masculine

In the accusative, masculine der changes to den, but feminine die stays die.

They are not the same thing.

• Jalousie usually means a blind with slats, like a Venetian blind
• Rollladen usually means a rolling shutter, often on the outside of a window

So the sentence is talking about two different window coverings.

This is a useful vocabulary distinction, because English sometimes uses blind, shade, or shutter more loosely.

Here lassen means something like leave or let remain.

The pattern is:

• lassen + object + state/position

So:

• den Rollladen = the object
• oben = the state or position

The idea is: I leave the roller shutter up.

This is a very common use of lassen in German.

Because oben describes the state of den Rollladen, and it normally comes after the object in this kind of structure.

So:

• ich lasse den Rollladen oben

means I leave the shutter up

German often puts this kind of word after the object:

• Ich lasse die Tür offen.
• Sie lässt das Fenster zu.
• Wir lassen das Licht an.

Because the subject is the same in both parts: ich.

German often leaves out the repeated subject after und when it is clearly understood:

• ... öffne ich die Jalousie und lasse den Rollladen oben.

This is completely normal. You could repeat ich, but it would usually sound less natural unless you wanted emphasis.

Yes. That is also correct.

The meaning stays basically the same, but the word order changes because now the main clause comes first:

• Ich öffne ...
• then the subordinate clause at the end: wenn die neue Glühbirne funktioniert

Notice that the verb in the wenn clause still goes to the end, no matter where that clause appears.

Because all German nouns are capitalized.

So in this sentence:

• Glühbirne
• Jalousie
• Rollladen

are all nouns, which is why they begin with capital letters.

This is a basic but very important difference from English.